U.S. President Joe Biden on Thursday urged American citizens to leave Ukraine immediately, and warned about a potential major conflict with Moscow should U.S. and Russian troops engage each other on the ground.

"American citizens should leave now," Biden said in an interview with NBC News, "We're dealing with one of the largest armies in the world. It's a very different situation and things could go crazy quickly."

The U.S. State Department has issued a new advisory urging Americans in Ukraine to leave the country at the earliest possibility, strengthening earlier warnings that urged U.S. citizens to "consider" such action.