The New START agreement is set to expire on Thursday, formally releasing both Moscow and Washington from a raft of restrictions on their nuclear arsenals.

Campaigners have warned allowing the treaty to lapse could unleash a new nuclear arms race.

In a call with China's President Xi Jinping, Putin said that Russia "will act in a measured manner and responsibly," after the treaty expires, Kremlin aide Yuri Ushakov said.

Putin offered a one-year extension of the pact in September.

US President Donald Trump said at the time it "sounds like a good idea", but the Kremlin says it has not received an answer.

Moscow remains "open to finding ways for dialogue and ensuring strategic stability", Usahkov added in a briefing to journalists, including from AFP.

The treaty, which included a monitoring mechanism, was signed in 2010 by then-Russian President Dmitry Medvedev and his US counterpart Barack Obama.

It limited each side's nuclear arsenal to 1,550 deployed strategic warheads and 800 missile launchers.

In 2023, Russia froze its participation in New START, but it has continued to voluntarily adhere to the limits.

Moscow last year tested its latest nuclear weapon carriers without atomic warheads, prompting Trump to announce he was moving two nuclear submarines closer to Russia.

The International Campaign to Abolish Nuclear Weapons (ICAN) called on Russia and the United States to commit to honour the New START limits while a new agreement was negotiated.

"Without New START, there is a real danger the new arms race will accelerate between the US and Russia -- more warheads, more delivery systems, more exercises -- and other nuclear-armed states will feel pressure to keep up," ICAN Executive Director Melissa Parke said Wednesday in a statement.