Putin is 'not bluffing' on Ukraine - ex NSC official

"My experience also of Putin is that he usually, if he threatens, he usually intends to act in some fashion," the former senior director for European and Russian affairs at the U.S. National Security Council said. "So we should be very careful about the idea of this being a bluff, but I know that many of our allies are a bit skeptical about what it is happening. We need to keep up our response as if we are taking this absolutely, one hundred per cent seriously," Hill added.

Earlier on Wednesday, the United States announced it will send nearly 3,000 troops to Poland and Romania to reinforce Eastern European NATO allies in the face of what Washington describes as a Russian threat to invade Ukraine.

Russia denies any plan to invade its neighbor but says it could take unspecified military measures if its demands are not met, including a promise by NATO never to admit Kyiv.