The United States is on track to open a new embassy in Tonga, the top US diplomat for east Asia says - part of efforts to step up its diplomatic presence in the Pacific region to counter China.

Daniel Kritenbrink told a subcommittee of the Senate Foreign Relations Committee the US was also continuing to engage with Vanuatu and Kiribati about opening proposed new embassies in those countries.

The State Department said in March it plans to open an embassy in Vanuatu.

The US has diplomatic relations with the South Pacific island nation, but these are currently handled by US diplomats based in Papua New Guinea.

The US reopened its embassy in the Solomon Islands this year after a 30-year absence.

Despite the diplomatic push, the Solomon Islands announced in March it had awarded a multi-million-dollar contract to a Chinese state company to upgrade an international port in Honiara.

The US and regional allies Australia and New Zealand have had concerns China has ambitions to build a naval base in the region since the Solomon Islands struck a security pact with Beijing last year.

Washington has also been working to renew agreements with the Marshall Islands, Palau and the Federated States of Micronesia under which it retains responsibility for the islands' defence and gains exclusive access to huge swaths of the Pacific.

The Biden administration is seeking $US7.1 billion ($A10.7 billion) from Congress across the next two decades for economic assistance to the three countries, funds seen as key to insulating them from growing Chinese influence.

The US is also planning a possible Biden stop in Papua New Guinea on May 22 as part of stepped-up engagement with the Pacific island region, according to officials familiar with the matter.