He spoke a day after Russian President Vladimir Putin announced the suspension of Moscow's participation in the New START arms treaty.

Biden made the brief comment in Warsaw ahead of a meeting with the leaders of several countries on NATO's eastern flank to discuss support for war-torn Ukraine.

When a journalist asked for a reaction to Russia's decision, Biden at first joked that he didn't "have time".

Then, after a pause, he said: "big mistake".

New START, the last nuclear arms control pact between Russia and the United States, commits them to limiting their stockpile of nuclear warheads.

Following Putin's announcement, Russian lawmakers on Wednesday unanimously approved the suspension of Moscow's participation in the treaty.

Russia's foreign ministry said it would still comply with the treaty until it expired in early 2026.

Russia's decision to suspend the treaty was "reversible" -- but Washington "must show political will", it added.

On Tuesday, US Secretary of State Antony Blinken called Russia's decision "deeply unfortunate and irresponsible" but said Washington was still willing to talk about the issue.

NATO Secretary General Jens Stoltenberg said Russia's move meant that "the whole arms control architecture has been dismantled".