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.

The International Campaign to Abolish Nuclear Weapons, winner of the 2017 Nobel Peace Prize, warned against allowing the treaty, which controls 87 percent of the world's nuclear weapons, to lapse.

"Without New START, there is a real danger the new arms race will accelerate between the US and Russia," ICAN chief Melissa Parke said in a statement.

That, she warned, would mean "more warheads, more delivery systems, more exercises -- and other nuclear-armed states will feel pressure to keep up".

"That makes every crisis more dangerous and increases the risk of mistakes and miscalculation."

Parke urged Washington and Moscow to "publicly commit to respect New START's limits while a new framework is negotiated".

Russian President Vladimir Putin already last September offered a one-year extension of the pact. While US President Donald Trump said at the time it "sounds like a good idea", the Kremlin says it has not received an answer.

If the treaty does lapse, "the two countries will be free to build and deploy more weapons, with the two sides unable to verify what the other is doing", ICAN pointed out.

"The arms race between the two countries, which already involves the development of new generations of weapons, could accelerate and the risk of an accidental or even intentional nuclear conflict will increase at a time when nuclear tensions are as high as they have ever been," it said.

Parke urged the United States and Russia to "restart serious disarmament talks and bring their warhead numbers down significantly.

"These weapons need to be eliminated before they are used again."