Antony Blinken has become the first US secretary of state to visit Tonga, dedicating a new US embassy and warning South Pacific nations about the perils of "predatory" Chinese investment.

As part of a drive to build Washington's influence across the region and to counter Beijing's growing clout, Blinken touched down in the capital Nuku'alofa on a diplomatic charm offensive on Wednesday.

"We're a Pacific nation", and "we very much see the future in the Indo-Pacific region", Blinken told his hosts.

"We really understand what is a priority for the people here," he added, citing issues like the climate crisis, development and illegal fishing.

Blinken also delivered a barbed warning about aid and investment from Beijing, saying it often comes with strings attached.

"As China's engagement in the region has grown, there has been some - from our perspective – increasingly problematic behaviour," Blinken said.

He claimed China had been behind "some predatory economic activities, and also investments that are done in a way that can actually undermine good governance and promote corruption".

Earlier this year, Micronesia's then-leader David Panuelo publicly accused China of engaging in harassment and espionage and bribing members of his cabinet.

Tonga's debt-laden government is seen as particularly vulnerable to economic pressure from China.

The island kingdom owes China's export bank around $130 million — almost a third of its GDP — which was loaned to help rebuild after rioters tore through the capital in 2006.

Repayments on that loan are scheduled to spike next year.

Diplomatic hub

Tonga, a Polynesian archipelago of about 100,000 people, is the latest in a string of Pacific island states being targeted in a renewed US diplomatic push.

The new US embassy in Nuku'alofa was officially opened in May, but Blinken's hosts said his visit signalled Washington's renewed interest in the region.

The United States has plans to open embassies in Vanuatu and Kiribati, and in February, restored its embassy in Solomon Islands after a 30-year hiatus.

The United States has been the dominant military force in the South Pacific since World War II. But the region is increasingly an arena for powers to compete for commercial, political and military influence.

China, the rising military force in the region, has also asserted itself through its expanding diplomatic reach, investment, police training and security deals.