President Donald Trump's administration has been insisting without success that its nemesis China enter the treaty, which has limited the United States and Russia to 1,550 nuclear warheads each and expires on February 5.

With three weeks to go before US elections in which Trump is trailing in polls, the administration indicated it would support preserving the treaty for an unspecified period.

"We are in fact willing to extend the New START treaty for some period of time provided that they, in return, agree to a limitation -- a freeze -- on their nuclear arsenal," US negotiator Marshall Billingslea said.

"We believe that there is an agreement in principle at the highest levels of our two governments," he said at the Heritage Foundation, a conservative think tank.

Billingslea cut short a trip to Asia last week to fly to Helsinki to see his Russian counterpart, Deputy Foreign Minister Sergei Ryabkov, saying that he sensed a breakthrough compromise.

But Ryabkov said that the US demand to freeze nuclear work in the interim was "an unacceptable proposition."

"If the Americans are in agreement with the documents that we handed them, a deal could even be reached tomorrow," Ryabkov said.

"But with so many differences, I cannot imagine on what basis our colleagues in Washington are putting out such theories," he told the Ria Novosti news agency.

Using similar language, Billingslea said that Russia needed to accept US proposals in his "gentlemen's agreement" with Ryabkov.

"We are ready to strike this deal. We can strike it tomorrow, in fact. But Moscow is going to have to show the political will to do so as well," Billingslea said.

- 'Serial treaty violators' -

At issue are US demands that Russia halt nuclear activities during the extension period and submit to verification.

Billingslea said that the United States would also accept reciprocal inspections.

"If we know anything about the Russians it is that they are serial treaty violators," he said.

The Trump administration has already left a landmark Cold War treaty that restricted intermediate-range nuclear forces, saying that Russia was in violation.

It also bolted from a treaty that allowed the two countries to fly over sensitive sites, with Trump reportedly piqued after a Russian spy jet flew over his Bedminister golf course in New Jersey.

Joe Biden, who leads Trump in all major polls, supports extending New START -- which would end days after he takes the oath of office.

Biden called the treaty -- which was negotiated by fellow Democrat Barack Obama and allows for an extension of up to five years -- an "anchor of strategic stability between the United States and Russia."

- Still pressuring China -

Billingslea said that the United States was still insisting on the participation of China -- whose nuclear program is quickly growing and faces no treaty restraints, even though it is still a fraction of the size of the Russian and US arsenals.

"Everything we agree with the Russians must be framed and must be formatted in a way that allows us to extend that arrangement to the Chinese when they are finally brought to the negotiating table," Billingslea said.

China's disarmament envoy, Li Song, on Tuesday reiterated Beijing's stance that it was unreasonable to join US-Russia talks, writing on Twitter: "There are only two largest nuclear arsenals on earth. Not three."

Russia -- which sees nuclear weapons as a key strategic asset as it is massively outspent on defense by Washington -- had 6,375 nuclear warheads at the start of the year, including those that are not deployed, according to the Stockholm International Peace Research Institute.

The United States had 5,800 while China was a distinct third with 320 warheads, according to the institute.

Billingslea, in a veiled swipe at US allies that back an extension of New START, urged more international pressure on China.

"The credibility of countries that profess to be at the leading edge of nuclear arms control is very much on the line. And we're taking note of those who have chosen to remain silent," he said.