The United States has accused Russia of violating the New Start treaty, the last major pillar of post cold-war nuclear arms control between the two countries, saying Moscow was refusing to allow inspection activities on its territory. The treaty came into force in 2011 and was extended in 2021 for five more years. It caps the number of strategic nuclear warheads that the United States and Russia can deploy, and the deployment of land- and submarine-based missiles and bombers to deliver them. The two countries, which during the cold war were constrained by a tangle of arms control agreements, still account together for about 90% of the world’s nuclear warheads. 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 Joe Biden’s administration to maintain and reach a follow-on agreement. “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 in emailed comments. Moscow in August suspended cooperation with inspections under the treaty, blaming travel restrictions imposed by Washington and its allies after Russian forces invaded neighbor Ukraine in February last year, but said it was still committed to complying with the provisions of the treaty. The state department spokesperson added that Russia had a “clear path” for returning to compliance by allowing inspection activities, and that Washington remains ready to work with Russia to fully implement the treaty. “The New Start treaty remains in the national security interests of the United States,” the spokesperson said. Talks between Moscow and Washington on resuming inspections under New Start were due to take place in November in Egypt, but Russia postponed them, accusing the United States of “toxicity and animosity”, and neither side has set a new date. On Monday, Russia told the United States that the treaty could expire in 2026 without a replacement because it said Washington was trying to inflict “strategic defeat” on Moscow in Ukraine. Asked if Moscow could envisage there being no nuclear arms control treaty after 2026, deputy foreign minister Sergei Ryabkov told the RIA state new agency: “This is quite a possible scenario.” The United States has supplied more than $27bn in security assistance to Ukraine since the invasion, including more than 1,600 Stinger anti-aircraft rocket systems, 8,500 Javelin anti-tank missile systems and 1 million 155mm artillery rounds. Russian president Vladimir Putin has issued thinly veiled threats to use nuclear weapons in Ukraine, reviving cold war era fears of an apocalyptic war. Shortly after taking office, Biden extended New Start by five years until 2026, giving time to negotiate while preserving what the Democratic administration sees as an important existing treaty. The previous administration of Donald Trump had ripped up previous arms control agreements and had been hesitant to preserve New Start in its current form, saying that any nuclear treaty must also include China, whose arsenal is rapidly growing but still significantly below those of Russia and the United States.