The US says it will join Australia in developing a naval base in Papua New Guinea (PNG), in an apparent move to curb China's growing influence.
Vice-President Mike Pence said the three countries would work together on the facility on Manus Island.
Australia announced last month that it would work with PNG to develop the island's Lombrum Naval Base.
Pence made the announcement on the sidelines of the Apec summit in the PNG capital, Port Moresby.
"The United States will partner with Papua New Guinea and Australia on their joint initiative at Lombrum Naval Base," he said.
"We will work with these two nations to protect sovereignty and maritime rights in the Pacific islands."
Lombrum, which was a major US naval base during World War Two, holds a strategic position overlooking busy trade routes.
The US and Australia have a shared concern over China's increasing influence in the region. Reports earlier this year that Beijing was looking to build a permanent military base in the South Pacific alarmed both countries.
Pence did not elaborate on US plans for the Manus Island base or whether US ships would be permanently stationed there.
He said the facility would show US commitment to an "open and free Indo-Pacific".
"Our commitment is to stand with countries across this region who are anxious to partner with us for security," he added.
Australian defence minister Christopher Pyne has said some Australian ships would probably be based permanently at Lombrum, broadcaster ABC reported.
Australia and the US are part of the "Five Eyes" Western intelligence alliance along with Canada, New Zealand and the UK, according to BBC.
Earlier this month Australian PM Scott Morrison announced a multi-billion dollar fund for Pacific island nations to build infrastructure - another apparent attempt to counter Beijing.
He said the project aimed to restore the Pacific to the "front and centre" of Australia's foreign outlook.
China has been providing millions of dollars in loans for infrastructure projects in the region and has become the second-largest donor of foreign aid there behind Australia.