Any use of chemical weapons would be a blatant and gross violation of international law - the ban on the use of chemical weapons, NATO Secretary-General Jens Stoltenberg told NBC.

"At the same time we know that Russia has used chemical agents in Europe before, against their own political opponents and Russia has been facilitating and supporting the Assad regime in Syria, where chemical weapons has been used," Stoltenberg said.

"We are very much aware of that we need to act in a way that prevents this conflict from going from being a very bloody, ugly, horrific conflict in Ukraine to something that turns out to be a full-fledged war between NATO and Russia, in Europe, and also potentially involving, of course, the United States directly," Stoltenberg added.