Russia’s president declared Tuesday that Moscow was suspending its participation in the New Start treaty  — the last remaining nuclear arms control pact with the United States — sharply upping the ante amid tensions with Washington over the fighting in Ukraine.

Speaking in his state-of-the-nation address, President Putin also said that Russia should stand ready to resume nuclear weapons tests if America does so, a move that would end a global ban on nuclear weapons tests in place since Cold War times.

Explaining his decision to suspend Russia’s obligations under New Start, Mr. Putin accused America and its NATO allies of openly declaring the goal of Russia’s defeat in Ukraine. “They want to inflict a ‘strategic defeat’ on us and try to get to our nuclear facilities at the same time,” he said.

Mr. Putin argued that while Washington has pushed for the resumption of inspections of Russian nuclear facilities under the treaty, NATO allies had helped Ukraine mount drone attacks on Russian air bases hosting nuclear-capable strategic bombers.

“The drones used for it were equipped and modernized with NATO’s expert assistance,” Mr. Putin said. “And now they want to inspect our defense facilities? In the conditions of today’s confrontation, it sounds like sheer nonsense.”

Mr. Putin emphasized, though, that Russia is suspending its involvement in New Start and not entirely withdrawing from the pact just yet.

The New Start treaty, signed in 2010 by Presidents Obama and Medvedev, limits each country to no more than 1,550 deployed nuclear warheads and 700 deployed missiles and bombers. The agreement envisages sweeping on-site inspections to verify compliance.

Just days before the treaty was due to expire in February 2021, Russia and the United States agreed to extend it for another five years. Russia and the U.S. have suspended mutual inspections under New Start since the beginning of the Covid-19 pandemic, but Moscow last fall refused to allow their resumption, raising uncertainty about the pact’s future. Russia also indefinitely postponed a planned round of consultations under the treaty.

In his long-delayed state-of-the-nation address Tuesday, Mr. Putin cast his country — and Ukraine — as victims of Western double-dealing and said it was Russia, not Ukraine, fighting for its very existence. “We aren’t fighting the Ukrainian people,” Mr. Putin said in a speech days before the war’s first anniversary on Friday. Ukraine “has become hostage of the Kyiv regime and its Western masters, which have effectively occupied the country.”

The speech reiterated a litany of grievances that the Russian leader has frequently offered as justification for the widely condemned war while vowing no military let-up in Ukrainian territories he has annexed illegally, apparently rejecting any peace overtures in a conflict that has reawakened fears of a new Cold War.

In the speech, Mr. Putin offered his own version of recent history, which discounted arguments by the Ukrainian government that it needed Western help to thwart a Russian military takeover. “Western elites aren’t trying to conceal their goals, to inflict a ‘strategic defeat’ to Russia,” Mr. Putin said in the speech broadcast by all state TV channels. “They intend to transform the local conflict into a global confrontation.”

He added that Russia was prepared to respond because “it will be a matter of our country’s existence.” He has repeatedly depicted NATO’s expansion to include countries close to Russia as an existential threat to his country.

Mr. Putin denied any wrongdoing, even as the Kremlin’s forces in Ukraine strike civilian targets, including hospitals, and are widely accused of war crimes. On the ground in Ukraine on Tuesday, grinding battles and shelling attacks continued in the east and the south of the country. At least six people were killed and seven more sustained injuries over the past 24 hours, Ukraine’s presidential office reported in the morning.

Many observers predicted Mr. Putin’s speech would address Moscow’s fallout with the West — and Mr. Putin began with strong words for those countries that have provided Kyiv with crucial military support. “It’s they who have started the war. And we are using force to end it,” Mr. Putin said before an audience of lawmakers, state officials, and soldiers who have fought in Ukraine.

Mr. Putin also accused the West of taking aim at Russian culture, religion, and values because it is aware that “it is impossible to defeat Russia on the battlefield.”

Likewise, he said Western sanctions would have no effect, saying they hadn’t “achieved anything and will not achieve anything.”