Secretary of State Rex Tillerson told reporters Monday evening that the poisoning of a former Russian spy in Britain last week "clearly came from Russia" and "certainly will trigger a response."
Tillerson added that he did not know whether the Kremlin had knowledge of the March 4 attack on Sergei Skripal and his daughter Yulia. But he did say that the poison could not have originated anywhere else and was "only in the hands of a very, very limited number of parties." He did not elaborate on what response might follow.
"There is never a justification for this type of attack – the attempted murder of a private citizen on the soil of a sovereign nation – and we are outraged that Russia appears to have again engaged in such behavior," Tillerson said in a statement. "From Ukraine to Syria – and now the U.K. – 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 spoke hours after British Prime Minister Theresa May told the House of Commons that Moscow was "highly likely" to blame for the attack in the southwestern city of Salisbury. Tillerson's statement said that he had "full confidence in the U.K.’s investigation" and its assessment of Russian responsibility.
May told lawmakers that unless her government received an explanation from Russia by end of Tuesday, Britain would consider the attack "an unlawful use of force by the Russian state against the United Kingdom."
"There can be no question of business as usual with Russia," she said, without saying what measures Britain might take.
Skripal, 66, and his 33-year-old daughter remained in critical condition. A police detective who came in contact with them is in serious but stable condition.
May said British scientists have determined that the Skripals were poisoned with Novichok, a class of nerve agent developed by the Soviet Union near the end of the Cold War.
She said it was "highly likely" the substance came from Russia, and there were two possible explanations.
"Either this was a direct act by the Russian state against our country, or the Russian government lost control of this potentially catastrophically damaging nerve agent and allowed it to get into the hands of others," she said.
May added that Russia must also "provide full and complete disclosure" of its Novichok program to the Organization for the Prohibition of Chemical Weapons, the oversight body for the international chemical weapons convention.
Russian Foreign Ministry spokeswoman Maria Zakharova dismissed May's allegations as a "circus show in in the British Parliament."
The White House said the use of the nerve agent "is an outrage" but wasn't ready to say that Russia was responsible. Spokeswoman Sarah Huckabee Sanders called the poisoning "reckless, indiscriminate and irresponsible," adding that the U.S. stands by its ally.
The Associated Press contributed to this report.