President Joe Biden said on Wednesday that Russian President Vladimir Putin made a “big mistake” by suspending his country’s participation in the the last remaining US-Russia nuclear arms control treaty.

The US President was in Poland to reassure eastern flank Nato allies that the US will remain by their sides amid the grinding Russian invasion of Ukraine.

In his first comments since Putin’s announcement on Tuesday, Biden condemned the Russian decision to pull back from the treaty, known as New START.

The move is expected to have an immediate impact on US visibility into Russian nuclear activities, but the pact was already on life support following Moscow’s cancellation late last year of talks that had been intended to salvage an agreement that both sides have accused the other of violating.

“It’s a big mistake,” Biden said.

The President’s comments came as he wrapped up a whirlwind, four-day visit to Poland and Ukraine with talks with leaders from the Bucharest Nine, a collection of nations in the most eastern parts of the Nato alliance that came together in response to Putin’s2014 annexation of Crimea from Ukraine.

He departed Poland on Wednesday evening to return to Washington.

The Bucharest Nine countries’ anxieties have remained heightened as the Ukraine war drags on, with many worrying that Putin could move to take military action against them next if he’s successful in Ukraine.

The alliance includes Bulgaria, the Czech Republic, Estonia, Hungary, Latvia, Lithuania, Poland, Romania and Slovakia.

“You’re the frontlines of our collective defence,” Biden said on Wednesday of the group.

“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.”

He pledged that Nato’s mutual-defence pact is “sacred” and that “we will defend literally every inch of Nato”.