The New START treaty, signed in 2010, caps to 1,550 the number of nuclear warheads that can be deployed by Moscow and Washington, who control the world's largest nuclear arsenals.

Negotiations to extend the treaty stalled as US President Donald Trump's administration insisted that China should join the deal, despite Beijing flatly rejecting the idea.

"Contacts now are indeed under way, energetic contacts," said Kremlin spokesman Dmitry Peskov, adding that extending the treaty was "in the interests of both countries and the whole world".

Last week, the Kremlin welcomed indications from the team of new US President Joe Biden that they would work towards a five-year extension.

Russia's Security Council confirmed on Monday that its chief Nikolai Patrushev had spoken to US National Security adviser Jake Sullivan about the mooted extension.

The Trump administration withdrew from several weapons treaties, including the Intermediate-Range Nuclear Forces (INF) treaty with Russia.

Relations between Moscow and Washington soured during Trump's tenure over allegations of cyber hacking and election meddling, and failure to negotiate an extension to New START would confirm the tattered state of bilateral ties.

Former Soviet leader Mikhail Gorbachev, who brokered the INF treaty with US president Ronald Reagan in 1987, urged both countries to extend New START and work together to agree further cuts to nuclear stockpiles.