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)"