US officials who visited the Solomon Islands are convinced "only a handful of people in a very small circle" have seen the final version of its controversial new security deal with China.

The high-level delegation arrived in the Pacific nation for talks with Prime Minister Manasseh Sogavare last Friday, three days after Beijing announced the deal had been signed.

A draft version of the pact, leaked on social media in March, prompted a flurry of lobbying by the United States and Australia, which are longtime allies of the Solomon Islands and fear an expansion of Chinese military reach in the region.

Quizzed Tuesday about whether the delegation had asked to see the deal during talks with Sogavare, US Assistant Secretary of State for East Asian and Pacific Affairs Daniel Kritenbrink said: "I think it's clear that only a handful of people in a very small circle have seen this agreement."

He added it was a source of concern that Sogavare had stated publicly he would "only share the details with China's permission".

Kritenbrink was critical of the "complete lack of transparency behind this agreement", saying other Pacific nations as well as "friends inside the Solomon Islands" were similarly concerned the deal had been inked behind closed doors.

"What precisely are the motivations behind the agreement? What exactly are China's objectives?" Kritenbrink asked.

"I think they are completely unclear, because this agreement has not been scrutinised, or viewed, or subject to any kind of consultation or approval process by anyone else."

Solomon Islands Prime Minister Manasseh Sogavare (L) has rebuffed fears that a leaked security pact with China would "destabilise" the region
Solomon Islands Prime Minister Manasseh Sogavare (L) has rebuffed fears that a leaked security pact with China would "destabilise" the region POOL via AFP / THOMAS PETER

Sogavare has repeatedly said the pact was related to domestic security issues and would not involve the building of a Chinese military base in the Solomons.

In Washington, Secretary of State Antony Blinken also voiced "concern" about the agreement.

Sogavare has "vowed publicly as well as privately that there would be no Chinese military base, no long-term presence, no power projection capability," Blinken told the Senate Foreign Relations Committee.

"We will be watching that very, very closely in the weeks and months ahead," Blinken said.

Provisions in the leaked draft deal have stoked fears of a more permanent military presence -- particularly measures that would allow Chinese naval deployments to the Pacific nation, which lies less than 2,000 kilometres (1,200 miles) from Australia.

The United States has said it will "respond accordingly" if China sets up a military base in the Solomons, but Kritenbrink declined to expound on the matter when pressed as to what that might entail.

He said that "we do know that the PRC is seeking to establish a more robust overseas logistics and basing infrastructure that would allow the PLA [People's Liberation Army] to project and sustain military power at greater distances".

The United States will "continue to monitor the situation closely", he said.