The Kremlin has said Russia will continue to act as a responsible nuclear power despite the expiry of the last remaining nuclear arms control treaty between Moscow and Washington, a development experts warn could open the door to a new global arms race.

The New START treaty expired on Thursday, bringing to an end more than five decades of restrictions on the strategic nuclear weapons of the United States and Russia.

“Today the day will end, and it [the treaty] will cease to have any effect,” Kremlin spokesperson Dmitry Peskov told reporters on Thursday. Arms control specialists had earlier assumed the agreement would expire at the end of Wednesday.

Russia had proposed that both sides voluntarily extend the treaty for one year to allow time for negotiations on a replacement agreement. Moscow said the proposal was never formally answered by US President Donald Trump.

“The agreement is coming to an end. We view this negatively and express our regret,” Peskov said, adding that the issue was discussed during a telephone call between Russian President Vladimir Putin and Chinese President Xi Jinping a day earlier.

“What happens next depends on how events unfold. In any case, the Russian Federation will maintain its responsible and attentive approach to strategic stability in the field of nuclear weapons and, as always, will be guided first and foremost by its national interests,” he said.

New START was first signed in Prague in 2010 by then US President Barack Obama and Russian President Dmitry Medvedev. It capped each side’s deployed strategic nuclear warheads at 1,550, representing a reduction of nearly 30 per cent from the limit agreed in 2002.

Deployed warheads refer to weapons in active service and ready for rapid use, as opposed to those held in storage or awaiting dismantlement.

The treaty also permitted on-site inspections of nuclear arsenals, although these inspections were suspended during the COVID-19 pandemic and have not resumed.