The web of arms control deals negotiated in the decades since the 1962 Cuban Missile Crisis, considered the closest the world ever came to intentional nuclear war, were aimed at reducing the chance of a catastrophic nuclear exchange.
Unless Washington and Moscow reach a last-minute understanding of some kind, the world's two biggest nuclear powers will be left without any limits for the first time in more than half a century when the New START treaty expires.
There was confusion about the exact time the 2010 deal would lapse, though arms control experts told Reuters they believed this would happen at 9am AEDT on Thursday.
Matt Korda, associate director for the Nuclear Information Project at the Federation of American Scientists, said if there was no agreement to extend its key provisions, neither Russia nor the US would be constrained if they wanted to add yet more warheads.
"Without the treaty, each side will be free to upload hundreds of additional warheads onto their deployed missiles and heavy bombers, roughly doubling the sizes of their currently deployed arsenals in the most maximalist scenario," he said.
Korda said it was important to recognise the expiry of New START did not necessarily mean an arms race given the cost of nuclear weapons.
US President Donald Trump has given different signals on arms control. He said last month that if the treaty expired, he would do a better agreement.
So far, Russian officials said, there has been no response from Washington on President Vladimir Putin's proposal to extend the limits of the treaty beyond expiry.
Total inventories of nuclear warheads declined to about 12,000 warheads in 2025 from a peak of more than 70,000 in 1986, but the United States and Russia are upgrading their weapons and China has more than doubled its arsenal over the past decade.
Arms control supporters in Moscow and Washington say the expiry of the treaty would not only remove limits on warheads but also damage confidence, trust and the ability to verify nuclear intentions.
Opponents of arms control on both sides say such benefits are nebulous at best and that such treaties hinder nuclear innovation by major powers, allow cheating and essentially narrow the room for manoeuvre of great powers.
Last year, Trump said he wanted China to be part of arms control and questioned why the US and Russia should build new nuclear weapons given that they had enough to destroy the world many times over.
"If there's ever a time when we need nuclear weapons like the kind of weapons that we're building and that Russia has and that China has to a lesser extent but will have, that's going to be a very sad day," he said in February last year.
"That's going to be probably oblivion."
19°C