"While the United States and the alliance seek to engage Russia, we must at the same time defend ourselves if Russia chooses to act contrary to international law," he said.
At the same time Mr. Mattis reminded:
"Just as we did throughout the darkest days of the Cold War, the United States remains willing to keep open political channels of cooperation and deescalate tensions.
"We remain open to opportunities to restore a cooperative relationship with Moscow, while being realistic in our expectations and ensuring our diplomats negotiate from a position of strength," he added.
Earlier it was reported that US will support build-up of Ukraine’s military potential. The first deputy head of Pentagon Robert Work met with the Foreign Minister of Ukraine Pavlo Klimkin.