NATO says any chemical attack by Russia in Ukraine would change the course of the war.
The military alliance is currently meeting and is expected to boost its troops in eastern Europe,
EU leaders - including Taoiseach Michael Martin - will hold talks later on whether more sanctions should be imposed on Russia.
NATO general secretary Jens Stoltenberg says their defences are "always ready" for any escalation.
"Any use of chemical weapons would fundamentally change the nature of the conflict.
"It would be a blatant violation of international law, and it would have widespread and severe consequences."
