Any use of chemical weapons by Russia in its war in Ukraine could cause contamination in NATO territory, Secretary General Jens Stoltenberg warned on Thursday.

“Any use of chemical weapons would fundamentally change the nature of the conflict – it would be a blatant violation of international law, and it will have widespread and severe consequences,” Stoltenberg said ahead of an extraordinary summit of NATO leaders.

“The seriousness of using chemical weapons, of course, becomes even more obvious knowing that there is a risk always for contamination … we can see the spread of chemical agents also into NATO territory,” he said.

Stoltenberg did not say how NATO would respond to such an attack.