Russian forces massing on the border with Ukraine have prompted the U.S. European Command to raise its watch level to its highest point of "potential imminent crisis", according to a New York Times report Tuesday night. On Wednesday, Chairman of the Joint Chiefs of Staff Gen. Mark Milley spoke to senior Russian Gen. Valery Gerasimov about the buildup, according to Russia.

Four Ukrainian soldiers were killed last week in an extended battle in the contested Donetsk Region against Russia-backed separatists. Russian forces that were thought to have gathered for training exercises near the border have stayed and grown in numbers, prompting concerns.