The Marine Corps this spring is sending its largest aircraft element to date -- four tilt-rotor Ospreys and five Super Cobra and four Huey helicopters, all out of Hawaii -- to the next $25 million rotation of Marines to northern Australia as the Corps continues to redistribute its forces around the Pacific.

The Marines are moving to four major forward areas of operation over the next decade -- Japan, Guam, Hawaii and Australia -- as part of a "distributed laydown" that seeks to deter growing threats in the vast Asia-Pacific region, particularly from from China and North Korea.

The Osprey, with its long range and speed, is seen as a key connector for the Marine Corps in its new island-hopping strategy.

In November 2011, President Barack Obama and Australian Prime Minister Julia Gillard announced that U.S. Marines would be sent on six-month rotational deployments to train with the Australian Defence Force in Darwin and elsewhere in the Northern Territory. The move puts the Marines closer to Southeast Asia and allows Australia, a key ally, to bolster its defense.

The presence was to grow to a 2,500-member Marine Air-Ground Task Force during the 2016-2017 time frame, but officials said the sixth iteration of the deployment will remain at 1,250 Marines who are expected to arrive in Australia in April.

Most of the Marines will be from the 3rd Battalion, 4th Marine Regiment, out of Camp Pendleton in California, Marine Corps Forces Pacific said.

According to Australia's Department of Defence, 200 Marines deployed in 2012, 250 in 2013, 1,150 in 2014, 1,150 in 2015 and 1,250 in 2016.

But the deployment number is stuck at 1,250 for the time being as funding problems continue to be worked out for the full Marine Air-Ground Task Force of 2,500 with aircraft, vehicles and other equipment.

"The Marine Corps position to date has been that we will not grow the force beyond 1,250 Marines in Australia until we get some top-line (overall Marine Corps budget) relief for funding for military construction," Craig Whelden, executive director of Marine Corps Forces Pacific at Camp H.M. Smith, said at a recent Chamber of Commerce Hawaii event.

Whelden added that "we've essentially tapped out what we can use of existing facilities and for sustainment." The deployment of 1,250 Marines costs about $25 million, he said. Marine Corps Forces Pacific is the tasking authority for the units participating in the rotations.

Australia and the United States in October agreed to a more than $1.5 billion cost-sharing agreement to improve infrastructure in northern Australia as well as pay for ongoing costs over the 25-year pact -- ending a disagreement over who would pay the tab, The Australian newspaper reported.

Australian media also said the Marine Corps rotational force is expected to double to its full strength of 2,500 by 2020.

The four Hawaii MV-22 Ospreys scheduled to make the deployment are from Marine Medium Tilt Rotor Squadron 268 (VMM-268), while the five AH-1W Super Cobra and four UH-1Y Venom helicopters are part of Marine Light Attack Attack Helicopter Squadron 367 (HMLA-367). The Aviation Combat Element of 13 aircraft will be hosted at Royal Australian Air Force Base Darwin.

By comparison, the Marines sent four UH-1Y helicopters for the rotational deployment in 2016 and four big CH-53E Super Stallions in 2015, according to Australia's Defence Department.

Whelden called the next contingent to deploy "a more diversified, a more capable force than we've had previously."

The "distributed laydown" in the Pacific, which dovetails with the Marine Corps' expeditionary nature and ability to rapidly move forces from one place to another, calls for moving more than 4,000 Marines from Okinawa, Japan, to Guam. Whelden said hundreds of millions of dollars already have been invested on Guam, with wharves to support amphibious ready group ships already completed and ramp and hangar work ongoing for Ospreys and the new F-35B Lightning II, which the Corps called the "world's first operational supersonic short takeoff and vertical landing fighter." "We aren't basing F-35s in Guam, but we train on Guam a lot," Whelden said. Ten of the stealth fighters this week were sent to Iwakuni, Japan, with six more expected to arrive this summer. Whelden said the relocation of about 2,700 Marines from Okinawa to Hawaii is about 10 years away.

During a visit to Sydney on Wednesday, the commander of the US Pacific Command, Admiral Harry Harris, signed a 2017 agreement for Australia to host Raptors, which will "send a strong signal about USA military presence in the region"

Euan Graham, the Lowy Institute's director of international security, described the presence of the F-22s as "pretty high-end coercive signalling to China". While the rotation of marines in Darwin got more attention, the stationing of planes was much more strategically significant, he said.

Barack Obama’s grand plan of using US marine exercises in the Northern Territory as a basis for a new military “pivot in Asia” has been finalised and locked in ahead of the Presidential elections next month.

Australia’s role in providing seasonal training base for 2,500 Marines — the original plan agreed to by President Obama and Julia Gillard in 2011 — was under threat from a dispute over cost sharing for facilities in the Northern Territory.

The dispute about shared costs was limiting the original plan to half the annual intake of 2,500 marines and undermining the effectiveness of the joint training scheme.

The end of Mr Obama’s Presidency, Donald Trump’s strident isolationism and threats from the new Philippines’ President Rodrgio Durtete to end US Marine exercises after this year and “break off” with the United States made finalisation of the original plan urgent.

Julie Bishop and Marise Payne were trying to organise an urgent meeting of US and Australian Foreign and Defence Ministers — Ausmin — in Sydney this month with US Secretary of State, John Kerry, and Defence Secretary, Ash Carter, to seal the deal before the election.

Malcolm Turnbull said on his trip to Washington DC two weeks ago, after meeting Mr Carter, that agreement would be reached but left the detail to the Defence Minister and Australian Defence Secretary, Dennis Richardson.

Senator Payne has been in Washington this week meeting Mr Carter and senior US Defence officials to discuss “the bilateral defence relationship and regional security and stability”.

Christopher Pyne, Defence Industry Minister, has also been in Washington this week in discussions over defence purchases and agreements.

A formal announcement of the final agreement is expected soon outside the auspices of an Ausmin meeting.

The US is facing increased pressure in South East Asia to withdraw troops from the Asia-Pacific realm and Australia’s role as the effective host to a battalion of US marines, who could be readily deployed to crises in the region, was even more crucial.

There were also fears that should Mr Trump become President, or even Hillary Clinton, a new administration would baulk at the costs and refuse to finalise the agreement stalling it at half its original strength and even going backwards.

Such a step would be a major strategic reversal in the region and deprive Australia of the opportunity to be a regional base for US military preparedness as well as cutting off millions of dollars from the Northern Territory economy.

As this stage the US Marine commitment for this year, the fifth rotation of US troops since the agreement began, of 1,250 troops is still only half the target President Obama and Ms Gillard envisaged in 2011.

The Abbott Government ratified the first stage of the agreement in 2014 and there were a scheduled 2,500 US Marines expected to be exercising near Darwin by 2016-17.

But because of an argument over cost sharing the troop rotation has stalled at half the target level. The fifth rotation of US marines for six months’ combined training with Australian and other forces in April this year was 1,250, half the final target after four previous rotations of less than half.

The Foreign Minister was confident an agreement would be reached in time soon after a series of meetings she had in Washington DC two weeks ago.

A formal national analysis of the “Force Posture” agreement found failure to implement it would “undermine Australia’s longstanding alliance with the US” — a view endorsed by the Parliamentary Treaties committee last year.

Although the training rotations at the Roberston Barracks in Darwin, which also involve aircraft and pre-positioned military hardware for a rapid deployment force, have begun the fight over the cost of deploying the Marines stalled the increase in numbers.

In April this year the fifth rotation began at Darwin Airport with the arrival of a US Galaxy C5 aircraft carrying CH-53E Super Stallion helicopters from Hawaii and the first of this year’s 1,250 US Marines.

US defence officials described the rotation as “part of the US rebalance to the Asia-Pacific region” and said it was not known when the target of 2,500 troops would be reached.

The officials said it was “not immediately clear when all 2,500 Marines are expected to rotate through Darwin” and that the decisions are made based on a number of variables including cost.

Mr Turnbull said on his arrival in New York for the UN General assembly the negotiations on cost-sharing for the rotation of up to 2,500 marines a year through Darwin was an ongoing negotiation. “I expect them to be resolved,” he said. “These are negotiations that are going on between officials.”

POTENTIALLY thousands of extra American sailors and Marines will flood through the Darwin port in coming years with a US General confirming a greater military presence in the South Pacific.

The head of US Marine Corps Force Command, Lieutenant General John Wissler, said the Pentagon planned to dispatch another three-ship amphibious ready group to the Asia-Pacific region as early as October 2018.

The Marine Corps Times reported that the ships would likely carry an embarked Marine Expeditionary Unit (MEU).

That would see an extra 4000 sailors and Marines in the region.

Lt-Gen Wissler said the goal of the MEU was to augment the number of Marines already coming to the Top End as part of the annual six-month Marine Rotational Force Darwin deployment.

This year, about 1250 Marine are deployed to Darwin, based mostly at Robertson Barracks about 20km from the Darwin CBD.

US and Australian officials hope to boost that number to 2500 by 2020.

“The actual basing (of the extra Marines and sailors) ... and all of those significant details are being worked out,” Lt-Gen Wissler said.

“That’s why it’s a (fiscal) 2019 problem because, as you can imagine, it’s a very complex undertaking.”

The Japan-based 31st Marine Expeditionary Unit presently conducts two 90-day patrols in the Pacific each year — one in spring and another in autumn — and respond to humanitarian crises in the region throughout the year.

Lt-Gen Wissler said the new arrangement would allow that unit to focus on north eastern Asia while the new ARG/MEU conducted two 90-day patrols in the South Pacific.

He said the Marine Corps has not determined whether the new MEU will come from the continental US, Hawaii or Japan.

Lieutenant Commander Matt Knight from the US Pacific Fleet said it is too soon to say which ships may be assigned to the MEU in fiscal 2019.

Former Chief of Naval Operations Admiral Jon Greenert said the amphibious assault ship America was a “prime candidate” to visit Australia as part of such a group.

“They will go into Darwin … and conduct on-load and off-load,” he told Navy Times in March 2015.

He said while the Marines trained in the Top End of Australia, the ships in the ARG could operate and hold exercises in South East Asia.

The U.S. and Australia have pushed back plans for the deployment of a 2,500-strong Marine Air Ground Task Force to Australia's Northern Territory until 2020 as they nail down the final details, including cost-sharing, Australian officials say.

The Marine Corps started building its presence in the port of Darwin in 2012 and grew its rotational force, which spends six months of the year training there, to just under 1,200 troops by 2014.

However, the growth of the force, which was to have included the task force within five years, has stalled with U.S. and Australian officials reporting only 1,250 Marines there this summer.

Australian Defence Force officials said in an email that they would work toward rotations of 2,500 Marines and equipment to Darwin by around 2020.

Australian national security consultant Ross Babbage said the two countries have been negotiating for three years over who would pay costs associated with the rotational force.

"There is no space on RAAF Darwin and Robertson Barracks (the facilities where Marines stay in Darwin) to facilitate that kind of growth (up to 2,500 Marines)"