US and Russian negotiators have agreed in principle to continue freezing their nuclear warhead stockpiles in a bid to salvage their last remaining arms control pact before it expires next year, a source has said.

The person said it is not yet clear if the agreement will succeed or translate into an extension of the New START treaty that expires in February, but if it works an agreement could be announced before the November 3 presidential election with an eye towards extending the accord and eventually bringing China into it, a long-time Trump administration demand.

The person said president Donald Trump and Russian leader Vladimir Putin have signed off on the freeze, but negotiators still need to iron out details, including compliance and verification issues. The person spoke to reporters after the last round of US-Russia arms control talks in Helsinki this week.

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The source said follow-up discussions between Washington and Moscow will take place next week. The Russians have been told that if a freeze is not agreed in the coming weeks, the US may harden its conditions.

Word of a potential freeze comes as Trump seeks foreign policy victories during a difficult re-election campaign against Joe Biden, who has vowed to extend New START even without Chinese participation.

An announcement ahead of the election could raise eyebrows, coming as US intelligence officials say Russia favours Trump and has been working to denigrate Biden.

The source said November 3 is not a final date for a freeze agreement with Moscow, and stressed that the Trump administration would continue to negotiate no matter the result of the election. But the person said the US would be looking for additional Russian concessions.