President Vladimir Putin took “unprecedented” post-Cold War action Sunday by ordering his nuclear deterrent forces to be on alert as international tensions over Russia’s invasion of Ukraine spiraled.

Putin said in a meeting of top officials shown Sunday on state TV that the move, which means the country’s nuclear weapons are prepared for increased readiness to launch, was in response to NATO powers’ making what he called “aggressive statements.”

Russia, like the U.S. and NATO, has thousands of nuclear warheads that it maintains as a deterrent to an attack.

The move is also a reaction to the West’s announcing hard-hitting financial sanctions against Russian businesses and key people, including Putin himself, he said in televised comments.

The Russian defense minister, Sergey Shoygu, and the chief of the military’s general staff have been ordered to put the nuclear deterrent forces in what was described as a “special regime of combat duty.”

Linda Thomas-Greenfield, the U.S. ambassador to the U.N., said Sunday morning on NBC News’ “Meet the Press” that “President Putin is continuing to escalate this war in a manner that is totally unacceptable.”

“And we have to continue to condemn his actions in the most strong, strongest possible way,” she added.

Daryl Kimball, the executive director of the Arms Control Association, a Washington nonprofit group, said Putin’s action is a modern first.

“This is unprecedented in the post-Cold War era,” Kimball said Sunday. “There has been no instance in which a U.S. or a Russian leader has raised the alert level of their nuclear forces in a middle of a crisis in order to try to coerce the other side’s behavior.”

Putin had promised a strong reaction to any Western power that tried to stand in his way in Ukraine. In a speech announcing his offensive Thursday, he warned that any attempt to interfere “will lead you to such consequences that you have never faced in your history.”

In February 2021, the U.S. renewed the New Strategic Arms Reduction Treaty, or New START, the nuclear arms control treaty with Russia, for five more years.

Signed in 2010 by Presidents Barack Obama and Dmitry Medvedev of Russia, the agreement placed the lowest limits in decades on U.S.- and Russian-deployed nuclear warheads and the land- and submarine-based missiles and bombers that deliver them.

Kimball said the escalated action by Putin shouldn’t be interpreted as a Russian threat to use nuclear weapons against the U.S.

“I think we need to understand that the risk of miscalculation and escalation is high,” Kimball said. “I don’t think we should look at this as a threat by Putin to use nuclear weapons against the United States, against Europe, against NATO. This is a dangerous moment in the crisis. It’s a point in which both sides needs to back down and move the word ‘nuclear’ from this equation.”