President Biden on Wednesday said that Russian President Vladimir Putin’s decision to suspend the New START nuclear arms treaty is the wrong move.

“Big mistake,” he told reporters in Warsaw, after previously saying he didn’t have time to discuss the issue.

The New START treaty, which is the only remaining nuclear arms control treaty with the U.S., is set to expire in 2026. Putin has raised further the prospect of nuclear escalation and the risk of a new nuclear arms race by announcing the suspension of Russia’s participation in it. 

The Kremlin made the announcement in remarks on Tuesday, just hours before Biden made remarks to mark the anniversary of the war in Ukraine. Putin in his speech also rehashed long-held grievances against the U.S., NATO and the government in Kyiv.

Biden on Wednesday spoke briefly to reporters when he arrived at the Presidential Palace in Warsaw for a meeting with the Bucharest Nine, which consists of members of NATO’s eastern flank: Bulgaria, the Czech Republic, Estonia, Hungary, Latvia, Lithuania, Poland, Romania and Slovakia. He was greeted by Polish President Andrzej Duda, President of Slovakia Zuzana Caputová, and President of Romania Klaus Iohannis.

Biden is set to depart Warsaw after the meeting and return to Washington.