Biden tells Putin that the U.S. and allies will respond "decisively" if Russia invades Ukraine

President Biden told Russian President Vladimir Putin in a phone call on Thursday that the U.S. and its allies will react "decisively" if Ukraine is invaded, White House press secretary Jen Psaki said. A senior administration official said Mr. Biden and Putin and agreed to a "sequence of strategic stability dialogue."

The call comes as Russia has built up troop levels along the border with Ukraine, and U.S. intelligence has warned Russia could invade Ukraine as soon as January. 

Meanwhile, a U.S. official confirmed to CBS News that an American spy plane flew a mission over Ukraine on Thursday. The flight was by a JSTARS aircraft, which is a radar plane specifically designed to track ground movements.