The treaty puts limits on the number of strategic nuclear warheads that the US and Russia can deploy, and the deployment of land- and submarine-based missiles and bombers to deliver them.

The United States has accused Russia of violating a key nuclear arms control agreement, the last major pillar of post-cold-war nuclear arms control between the two countries, saying Moscow was refusing to allow inspections of its nuclear facilities.

“Russia is not complying with its obligation under the New START Treaty to facilitate inspection activities on its territory. Russia’s refusal to facilitate inspection activities prevents the United States from exercising important rights under the treaty and threatens the viability of US-Russian nuclear arms control,” a State Department spokesperson said Tuesday.

“Russia has also failed to comply with the New START Treaty obligation to convene a session of the Bilateral Consultative Commission in accordance with the treaty-mandated timeline,” the spokesperson added.

The treaty, which came into force in 2011 and was extended in 2021 for five more years, is the only agreement left regulating the world’s two largest nuclear arsenals, which during the cold war were constrained by a tangle of arms control agreements.

Under it, both countries are permitted to conduct inspections of each other’s weapons sites, but due to the Covid-19 pandemic, inspections have been halted since 2020.

Washington has been keen to preserve the treaty but ties with Moscow are the worst in decades over Russia’s invasion of Ukraine, an element that could complicate attempts by US President Joe Biden’s administration to maintain and reach a follow-on agreement.

Russia last August suspended cooperation with inspections under the treaty, blaming travel restrictions imposed by the US and its allies after Russian forces invaded Ukraine last February, but said it was still committed to complying with the provisions of the treaty.

A session of the Bilateral Consultative Commission on the treaty was slated to meet in November but was abruptly called off by Russia, which accused the US of “toxicity and animosity”. Neither side has set a new date.

The Tuesday statement noted that Russia had a “clear path” for returning to compliance by allowing inspection activities and the US remains ready to work with the country to fully implement the treaty.

“The New Start treaty remains in the national security interests of the United States,” the spokesperson said.

Russia told the US Monday that the treaty could expire in 2026 without a replacement because it said Washington was trying to inflict “strategic defeat” on Moscow in Ukraine.