President Biden poses during a group photo with the Polish President Andrzej Duda and Romanian President Klaus Iohannis at the Presidential Palace in Warsaw on Feb. 22, 2023. MANDEL NGAN/AFP VIA GETTY IMAGES

President Biden said Wednesday that it is a "big mistake" for Russia to suspend its participation in the New START nuclear arms control pact, the last remaining nuclear treaty between the U.S. and the Kremlin.

Mr. Biden, who was in Warsaw, Poland, to meet with Eastern European NATO allies, was asked for his reaction to the decision announced by Russian President Vladimir Putin, and told reporters such such a step was a "big mistake."

Putin announced Russia would be pausing its participation in the nuclear arms control treaty during his state of the nation address Tuesday, raising tensions between the U.S. and Russia amid the ongoing war in Ukraine. The Russian leader also said the country should be prepared to resume nuclear weapons tests if the U.S. did so first.

Mr. Biden elaborated on that response in an interview with ABC News' David Muir Thursday.

"It's a big mistake, to do that. Not very responsible. ... But I don't read into that that he's thinking of using nuclear weapons or anything like that."

Hours after Putin's speech, Russia's Ministry of Foreign Affairs said the country would still comply with the limits on offensive weapons imposed by the treaty and continue to exchange information with the U.S. about ballistic missile tests, in order to "maintain the necessary level of predictability and stability in the nuclear missile area."

Secretary of State Antony Blinken condemned Putin's decision to step back from the pact, calling it "deeply unfortunate and irresponsible."

The New START treaty was signed in 2010 by then-President Barack Obama and Russian President Dmitry Medvedev. The treaty limits each country to no more than 1,550 deployed nuclear warheads and 700 deployed intercontinental ballistic missiles and heavy bombers. It also includes a number of verification measures, including 18 on-site inspections annually for U.S. and Russian teams.

The pact was set to expire in February 2021, but the U.S. and Russia struck a deal to extend it through February 2026.

The announcement from the Russian leader came a day after Mr. Biden made a secret, hourslong visit to Kyiv and met with Ukrainian President Volodymyr Zelenskyy. The trip, taking place nearly one year since Putin launched the full-scale invasion of Ukraine, was a significant show of support for the country in its continued efforts to combat Russia's aggression.

Mr. Biden reiterated that the U.S. would continue to offer steadfast support of Ukraine, including through another batch of weapons and emergency assistance.

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