Both Washington and Moscow cast the extension as a victory, saying it would provide stability and transparency.
The United States and Russia have finalised an agreement to extend until 2026 a treaty limiting their stockpiles of nuclear weapons.
The New Strategic Arms Reduction Treaty (New START Treaty), which was due to expire on Friday, imposes limits on Russian and US intercontinental missiles and bombers, but does not cover new types of weapons.
Both Washington and Moscow cast the extension as a victory, saying it would provide stability and transparency on nuclear issues while acknowledging some of their disagreements.
“Even as we work with Russia to advance U.S. interests, so too will we work to hold Russia to account for adversarial actions as well as its human rights abuses,” Secretary of State Antony Blinken said in a statement.
Blinken said Washington would use the five-year extension to pursue additional limits on all of Russia’s nuclear weapons.
“President [Joe] Biden has made clear that the New START Treaty extension is only the beginning of our efforts to address 21st Century security challenges,” he said.
Russia and the US last week announced plans to extend the agreement, even as the Biden administration has stepped up criticism of Russia over the jailing of opposition leader Alexey Navalny, its alleged involvement in a massive US government hack and other issues.
The New START Treaty, which went into effect in 2011, limits the US and Russia to deploying no more than 1,550 strategic nuclear warheads each and imposes restrictions on the land and submarine-based missiles and bombers to deliver them.
The outgoing Trump administration made a late bid to extend the treaty, but Russia rejected its conditions.
Both houses of the Russian parliament voted unanimously last month for the extension, and President Vladimir Putin signed the bill.
That was after Biden and Putin talked and agreed on the extension, part of a quick round of diplomacy by the new US administration. The extension does not require formal approval by the US Congress.
In a statement, the Russian foreign ministry said the diplomatic notes needed to formally extend New START had been exchanged on Wednesday. A US official confirmed the exchange.
The Russian statement did not address wider disputes with the US, including Washington’s condemnation of Moscow’s 2014 annexation of Crimea from Ukraine.
But Russia’s foreign ministry criticised the US for pulling out of other arms control pacts under former President Donald Trump.
Trump had tried and failed to tie the New START Treaty to three-way talks with China covering its far smaller arsenal.
“We expect that the (New START) understanding … would allow (us) to leave behind the trend towards dismantling of arms control and non-proliferation mechanisms, so prevalent in recent years due to US destructive policies,” the Russian ministry said.
Tom Collina of Ploughshares Fund, which advocates for the elimination of nuclear weapons, said Russia’s priority in any new accord would be dealing with the threat it sees to its long-range strategic nuclear arsenal from US missile defences.
Washington, for its part, likely will seek to limit Moscow’s vast short-range nuclear arsenal, Collina told the Reuters news agency.