The United States said Tuesday that Russia was not complying with New START, the last remaining arms control treaty between the world's two main nuclear powers, as tensions soar over the Ukraine war.

Responding to a request from Congress, the State Department faulted Russia for suspending inspections and canceling talks but did not accuse Moscow of expanding nuclear warheads beyond agreed limits.

"Russia is not complying with its obligation under the New START Treaty to facilitate inspection activities on its territory," a State Department spokesperson said, charging that Moscow's refusal "threatens the viability of US-Russian nuclear arms control."

"Russia has a clear path for returning to full compliance. All Russia needs to do is allow inspection activities on its territory, just as it did for years under the New START Treaty, and meet in a session of the Bilateral Consultative Commission," he said, referring to the formal talks set up under the treaty.

"There is nothing preventing Russian inspectors from traveling to the United States and conducting inspections."

Moscow announced in early August that it was suspending US inspections of its military sites under New START. It said it was responding to American obstruction of inspections by Russia, a charge denied by Washington.

Diplomacy between the two powers has ground to a bare minimum over the past year as the United States leads a drive to punish Russia economically for its war against Ukraine and arm Kyiv with billions of dollars in weapons.

Russian President Vladimir Putin has issued thinly veiled threats to use nuclear weapons in Ukraine, reviving Cold War era fears of an apocalyptic war.

Russia indefinitely postponed talks under New START that had been due to start on November 29 in Cairo, accusing the United States of "toxicity and animosity."

- 'Make the world safer' -

President Joe Biden shortly after taking office 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.

The Biden administration indicated that it wanted to preserve New START. The State Department spokesperson said the treaty was meant "to make the world safer."

Republican lawmakers, who took control of the House of Representatives in January, had asked Secretary of State Antony Blinken to report by Tuesday whether Russia was in violation.

A group of Republicans active on defense policy responded that Biden had "naively" extended New START and said that Russia "cannot be trusted to abide by any international agreement."

"We urge President Biden to direct the Department of Defense to prepare for a future where Russia may deploy large numbers of warheads, well in excess of New START treaty limits," said a statement by Republicans including Representative Mike Rogers, chairman of the House Armed Services Committee.

New START, signed by then president Barack Obama in 2010 when relations were warmer, restricted Russia and the United States to a maximum of 1,550 deployed strategic nuclear warheads each -- a reduction of nearly 30 percent from the previous limit set in 2002.

It also limits the number of launchers and heavy bombers to 800, still easily enough to destroy Earth.