The United States joined Russia in extending the two countries’ last remaining treaty limiting their stockpiles of nuclear weapons, two days before the pact was set to expire.
Secretary of State Antony Blinken said in a statement that the US would use the five years of the New START treaty’s renewal to pursue limits on all of Russia’s nuclear weapons.
This comes after the Trump administration pulled out of two other such deals, as part of a broad withdrawal from international accords.
Last week, the countries announced plans to extend the agreement, even as the Biden administration stepped up criticism of Russia over the jailing of opposition leader Alexei Navalny, its involvement in a massive hack and other issues.
Mr Blinken said: “Especially during times of tension, verifiable limits on Russia’s intercontinental-range nuclear weapons are vitally important.
“Extending the New START treaty makes the United States, US allies and partners, and the world safer.
“An unconstrained nuclear competition would endanger us all.”
The treaty, signed in 2010 by then-US president Barack Obama and then-Russian president Dmitry Medvedev, limits the number of US and Russian strategic nuclear weapons.
The outgoing Trump administration made a late bid to extend the treaty, but Russia rejected its conditions.
The treaty was due to expire on Friday.
Both houses of the Russian parliament voted unanimously last month for the extension, and President Vladimir Putin signed the bill.
That was after US president Joe Biden and Mr Putin talked and agreed on the extension, part of a quick round of diplomacy by the less-than-month-old US administration to keep the treaty going.
The extension does not require formal congressional approval in the United States.
The Biden administration will also work on control measures for China’s smaller but growing arsenal of nuclear warheads, Mr Blinken said.