Warsaw, Poland — Russian President Vladimir Putin’s decision to pull out of the country’s sole remaining nuclear arms control treaty with the US was a "big mistake," President Joe Biden said Wednesday, the latest volley in a diplomatic showdown between Washington and Moscow as the president wraps up a four-day trip to the region.

President Biden weighed in on the Kremlin’s announcement as he arrived at the presidential palace in Warsaw, where he met with leaders of nations from the NATO alliance’s eastern flank amid unease over Russia’s ongoing assault in Ukraine.

The Bucharest Nine – Bulgaria, the Czech Republic, Estonia, Hungary, Latvia, Lithuania, Poland, Romania, and Slovakia – was formed in the wake of Putin’s 2014 annexation of Crimea. Leaders worry a Russian victory in Ukraine could embolden Putin to invade other countries on the eastern edge of Europe.

Putin announced this week during his annual parliamentary address that he was suspending cooperation with the New START nuclear nonproliferation agreement, ratcheting up tensions with Washington as he faulted Ukraine and Western allies for starting the war. The Russian leader’s announcement came hours before Biden delivered remarks at Warsaw’s Royal Castle to mark the anniversary of Moscow’s assault on Ukraine a year ago.

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"It was a big mistake," said Biden, bearing a black smudge on his forehead to mark Ash Wednesday, the first day of Lent.

Biden extended the New START treaty with Russia for an additional five years when he took office, arguing it was in the best security interests of both countries.

The meeting puts an exclamation point on Biden’s trip to Poland and surprise stop in Ukraine to highlight the US and Western allies’ continued commitment to support Kyiv in the face of Russian aggression as the war stretches into a second year.

"You’re the front lines of our collective defense," Biden told the group of leaders. "And you know, better than anyone, what’s at stake in this conflict. Not just for Ukraine, but for the freedom of democracies throughout Europe and around the world."

Romanian President Klaus Iohannis said the war has "brought nothing but suffering and despair, killing and displacing of millions of Ukrainians."

"We the leaders of the eastern flank have the duty to stand firm in defense of our peace," Iohannis told the group.