The head of the US Navy's Indo-Pacific Command is urging Congress to build a new missile defence base in the Pacific as part of a larger strategy to counter China's military threat to the region.

The Pentagon is seeking $US77 million ($98 million) to build a permanent "land-based integrated air and missile defence system and associated weapon delivery system on Guam", as outlined in the Indo-Pacific Deterrence Act.

In addition to the defence system, which is designed to track down and destroy missiles before they do harm, the US is considering an increase to its ground-based missiles on Guam that could fire on targets over 500 kilometres away.

Almost a third of Guam's land is controlled by the US military, and Ms McManus fears its presence is destroying the island's natural environment.