Washington DC - The last nuclear treaty between Russia and the US expired on Thursday, abruptly ending decades of restrictions on how many warheads the two top powers can deploy and triggering fears of a global arms race.

New START, signed during a warmer period of relations, ended with the turn of the calendar to February 5, with President Donald Trump not following up on Russian counterpart Vladimir Putin's proposal to extend the treaty's limits for one year.

UN Secretary-General Antonio Guterres called it a "grave moment for international peace and security" and urged Washington and Moscow to head quickly to the negotiating table.

"For the first time in more than half a century, we face a world without any binding limits on the strategic nuclear arsenals of the Russian Federation and the United States of America," Guterres said in a statement, which considered the treaty over at midnight GMT or 7 PM in New York.

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"This dissolution of decades of achievement could not come at a worse time – the risk of a nuclear weapon being used is the highest in decades," he said, after Russian suggestions of using tactical nuclear weapons early in the Ukraine war.

Pope Leo XIV said each side needed to do "everything possible" to avert a new arms race.

"I urge you not to abandon this instrument without seeking to ensure that it is followed up in a concrete and effective manner," the American-born pope said at his weekly general audience.

Russia's foreign ministry said it considered that both countries are "no longer bound by any obligations or symmetrical declarations within the context of the treaty."

"The Russian Federation intends to act responsibly and prudently," it added, but warned it was ready to take "decisive" countermeasures if its national security is threatened.