The New START agreement will end Thursday, formally releasing both Moscow and Washington from a raft of restrictions on their nuclear arsenals.

"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.

The UN secretary-general added that New START and other arms control treaties had "drastically improved the security of all peoples."

"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, without giving more details.

Guterres urged Washington and Moscow "to return to the negotiating table without delay and to agree upon a successor framework."

Russia and the United States together control more than 80 percent of the world's nuclear warheads but arms agreements have been withering away.

New START, first signed in 2010, limited each side's nuclear arsenal to 1,550 deployed strategic warheads -- a reduction of nearly 30 percent from the previous limit set in 2002.

It also allowed each side to conduct on-site inspections of the other's nuclear arsenal, although these were suspended during the Covid-19 pandemic and have not resumed since.