Secretary of State Rex Tillerson said Monday that the poisoning of an ex-Russian spy in Britain “clearly came from Russia” and “certainly will trigger a response.”

Tillerson said in a statement that he doesn’t know if the Kremlin was aware of the poisoning, but that the nerve agent used in the attack could not have come from any other country.

He called the attack “a really egregious act” and said it was “almost beyond comprehension” that officials of any government would use the poison in a public location.

“From Ukraine to Syria — and now the [United Kingdom] — Russia continues to be an irresponsible force of instability in the world, acting with open disregard for the sovereignty of other states and the life of their citizens,” Tillerson said.

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Ex-Russian spy Sergei Skripal and his daughter were found to have been poisoned with a nerve agent in Britain last week.

British Prime Minister Theresa May said Monday that it was “highly likely” that Russia was responsible for the poisoning. She gave Russian officials until Tuesday to respond to her claims.

The White House on Monday voiced support for Britain but declined to say directly whether Moscow was responsible for the poisoning.

“Right now we are standing with our U.K. ally. I think they’re still working through even some of the details of that,” press secretary Sarah Huckabee Sanders said at a press briefing. “And we’re going to continue to work with the U.K., and we certainly stand with them throughout this process.”

Russian officials have dismissed the British allegations as “provocation” and “nonsense.”