The United States and Russia have both declared that they complied with a 2010 arms-control treaty limiting the two countries' massive nuclear arsenals, as Moscow criticized President Donald Trump's new nuclear policy.
February 5 was the deadline for the two countries to comply with the New START deal, which restricts each to 1,550 deployed strategic nuclear warheads and 700 deployed intercontinental ballistic missiles and strategic bombers.
With U.S.-Russian relations plummeting to levels not seen since the Cold War, and both countries moving to upgrade their arsenals, the status of the treaty has been closely watched by arms-control experts.
Some of them fear another important arms-control agreement -- the 1987 Intermediate-Range Nuclear Forces treaty -- is on the verge of collapse, amid U.S. accusations that Russia has deployed a prohibited missile system.
And several influential Republican lawmakers in Congress who have criticized Trump's predecessor for its response to the alleged Russian violations suggested that they would not vote to extend New START when the treaty expires in 2021.
The State Department said in a statement on February 5 that Washington fulfilled its commitments under New START in August 2017.
Adherence to the treaty was "critically important at a time when trust in the relationship has deteriorated, and the threat of miscalculation and misperception has risen," department spokeswoman Heather Nauert said.
Russia has "repeatedly stated its commitment" to the treaty, including meeting the central limits, and we expect our upcoming data exchange under the treaty to reaffirm that commitment," she said.
The Russian Foreign Ministry, meanwhile, said in a statement that it now had 1,444 strategic nuclear warheads and had deployed 527 intercontinental ballistic missiles and strategic bombers.
Over the weekend, the ministry took aim at the newly announced U.S. Nuclear Posture Review, a policy document released on February 2 that lays out the thinking of the Trump administration on nuclear weapons use.
The policy says the United States will modify some of its existing long-range ballistic missiles carried by submarines to fit them with smaller-yield warheads. It says the administration will develop a nuclear-armed cruise missile that can be launched by submarines.
The new policy also points to a Russian doctrine known as "escalate to de-escalate." That doctrine, first articulated in 2000 not long after President Vladimir Putin took office, stipulates that Moscow would use or threaten to use smaller-yield nuclear weapons in a conventional conflict in Europe if Russia felt it faced an existential conventional threat.
Moscow suggested the new U.S. policy was dangerous because it would make it difficult to confirm that a reconfigured U.S. weapon would not be carrying nuclear weapons, raising the prospect of a Russian nuclear response.
"Readiness to use nuclear weapons to prevent Russia from using its nuclear arsenal, expressed in the new Nuclear Posture Review, amounts to putting in question our right to defend ourselves against an aggression that threatens the country's survival," the ministry said in a statement.