U.S. President Joe Biden said Monday that North Korea sending thousands of troops to Russia is a “very dangerous” development.

Biden made the comment when addressing reporters after casting his ballot in the U.S. presidential election in early voting in New Castle, Delaware.

It is the first time Biden has spoken publicly about the presence of North Korean troops in Russia. 

The U.S. president took the opportunity to criticize his predecessor, former President Donald Trump, for talking to Russian President Vladimir Putin and “discussing what should be done.”