He said this at the Bucharest Nine Summit in Warsaw, CNN reports.
When Biden was asked what he thought of Putin's statement, the American leader replied: “I don't have time.”
He then paused and said “a big mistake” before going to take a picture with the other leaders.
The Bucharest Nine was formed in 2015 after Russia's annexation of Crimea, and includes Bulgaria, the Czech Republic, Estonia, Hungary, Latvia, Lithuania, Poland, Romania, and Slovakia.
On February 21, President Vladimir Putin announced that Russia had decided to suspend its participation in New START.