Russian President Vladimir Putin must not cross the "very important line" of using nuclear weapons of any kind in Ukraine, NATO General Secretary Jens Stoltenberg said Thursday.

Stoltenberg made the comments following a meeting with NATO's Nuclear Planning Group on Thursday. Both NATO and Russia are expected to move forward with pre-planned nuclear testing this month, though NATO does not plan to use live bombs.

Putin has repeatedly claimed in recent weeks that Russia would resort to nuclear weapons if the country's territory is threatened.

"We will not go into exactly how we will respond, but of course this will fundamentally change the nature of the conflict. It will mean that a very important line has been crossed," Stoltenberg said.

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Jens Stoltenberg

NATO Secretary General Jens Stoltenberg speaks during a media conference after a meeting of the NATO-Ukraine Commission at NATO headquarters in Brussels on Feb. 22, 2022. (AP Photo/Olivier Matthys)

Vladimir Putin stands at a podium

Vladimir Putin delivers an address flanked by men in military uniforms. (Pavel Bednyakov, Sputnik, Kremlin Pool Photo via AP)

"Even any use of a smaller nuclear weapon will be a very serious thing, fundamentally changing the nature of the war in Ukraine, and of course that would have consequences," he added.

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Ukrainian President Volodymyr Zelenskyy argues that Putin is already trying to prepare Russian citizens for nuclear war.

"They begin to prepare their society," Zelenskyy last week. "That's very dangerous. They are not ready to do it, to use it, but they begin to communicate."

"They don't know whether they'll use or not use it. I think it's dangerous to even speak about it," he added.

Zelenskyy Ukraine Biden phone call

Ukrainian President Volodymyr Zelenskyy speaks with President Biden on Aug. 25, 2022. (Ukrainian President Volodymyr Zelenskyy)

President Biden warned last week that Putin is "not joking when he talks about potential use of tactical nuclear weapons or biological or chemical weapons because his military is, you might say, significantly underperforming."

"We have not faced the prospect of Armageddon since Kennedy and the Cuban Missile Crisis," he added, raising eyebrows across the country.

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White House press secretary Karine Jean-Pierre later clarified that the U.S. has seen no indication that Russia is preparing to imminently use nuclear weapons.

"The kind of irresponsible rhetoric we have seen is no way for the leader of a nuclear-armed state to speak, and that’s what the president was making very clear," she said last week.