The Kremlin said on Tuesday that allowing the New START nuclear treaty with the United States to expire next February would be fraught with risks for international security.
In a call with reporters, Kremlin spokesman Dmitry Peskov said that Russia would have to take unspecified measures if the US did not agree to President Vladimir Putin's proposal on Monday that both sides should adhere to the treaty's limits on deployed strategic nuclear weapons for another year.
New START, the last nuclear agreement between the two countries, is due to expire on February 5, and Peskov said it would be "virtually impossible" to negotiate a successor treaty before then - hence Putin's suggestion to stick to its prescribed limits on nuclear warheads.
"Time...is running out, and we are truly on the threshold of a situation where we could be left without any bilateral documents regulating strategic stability and security, which, of course, is fraught with great dangers from a global perspective," Peskov said.
He said Putin's initiative had not been discussed in advance with US President Donald Trump.
The White House said on Monday that Putin's proposal sounded "pretty good", and Trump would address it.
Keeping the treaty limits in force would enable both sides to avoid - or at least postpone – an expensive arms race that nuclear experts say would likely ensue if New START were to lapse altogether.
The two presidents could present it as a positive diplomatic achievement after months of telephone contacts and a summit in Alaska in August that failed to yield progress on ending Russia's war in Ukraine.
Putin said on Monday that Russia would only extend its observance of the treaty's limits if the Americans did the same.
"If they are not complied with on the other side, then, of course, measures will have to be taken," Peskov said, without elaborating on what steps Russia might then take.
He said it was not clear when the next contact between Putin and Trump might take place.
Russia and the United States have by far the biggest nuclear arsenals in the world. New START caps the number of deployed strategic nuclear warheads at 1,550 and the number of delivery vehicles - missiles, submarines and bomber planes - at 700 on each side.
(Reporting by Dmitry Antonov, Writing by Mark Trevelyan and Felix Light Editing by Andrew Osborn)