U.S. Secretary of State Rex Tillerson said the United States is "outraged" over what he said was Russia's likely poisoning of former spy Sergei Skripal in Britain, and warned that Moscow "certainly will" face an allied response over the matter.
"We have full confidence in the U.K.'s investigation and its assessment that Russia was likely responsible for the nerve agent attack that took place in Salisbury last week," Tillerson said in a statement late on March 12 hours after British Prime Minister said it was "highly likely" Russia was behind the March 4 attack on Skripal and his daughter, both of whom were still hospitalized and in critical condition.
"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.
"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," he said.
"We agree that those responsible -- both those who committed the crime and those who ordered it -- must face appropriately serious consequences," Tillerson said. "We stand in solidarity with our allies in the United Kingdom and will continue to coordinate closely our responses."
Tillerson's vow of allied action in response to what he said was the use of a military-grade nerve agent called Novichok that was developed by Russia during the Cold War was echoed by other British allies.
NATO Secretary General Jens Stoltenberg said "the U.K. is a highly valued ally, and this incident is of great concern to NATO... The use of any nerve agent is horrendous and completely unacceptable."
In a phone call with May, French President Emmanuel Macron "offered his solidarity with the U.K." Downing Street said.
"They discussed the wide pattern of aggressive Russian behavior and agreed that it would be important to continue to act in concert with allies to address it," it said.
British officials called in Moscow's ambassador in London on March 12 and demanded that he explain by the end of the day on March 13 how the deadly chemical made its way into Britain to be used against a former spy.
Skripal, a retired Russian military intelligence colonel, was convicted by a Moscow military court in 2006 of "high treason" for passing secrets to Britain's Secret Intelligence Service. He was one of four Russian prisoners released in 2010 in exchange for 10 Russian sleeper agents detained in the United States.
May said there were only two possible reasons for the detection of the Russian-made nerve agent: that it was used in "a direct action by the Russian state," or because of Russia "losing control" of its stocks of the deadly chemical.
"Should there be no credible response, we will conclude that this action amounts to an unlawful use of force by the Russian state against the United Kingdom," May said, calling the attack a "reckless and despicable act."
"We will not tolerate such a brazen attempt to murder innocent civilians on our soil," she added.
The Russian Foreign Ministry was quick to denounce May’s accusations, saying in a statement that they were politically motivated and based on a provocation.
"It is a circus show in the British parliament," the TASS news agency quoted Foreign Ministry spokeswoman Maria Zakharova as saying.
While May on May 12 did not repeat previous British threats to boycott the soccer World Cup in Russia this summer over the incident, the Russian ministry accused Britain of manufacturing the Skripal poisoning as an excuse to skip the games.
Before making his statement, Tillerson called his British counterpart Foreign Secretary Boris Johnson to discuss the poisoning.
Speaking to reporters during a flight back to Washington from Africa, Tillerson agreed with May's assessment that the attackers had used a nerve agent developed by Russia.
"It appears that it clearly came from Russia. Whether it came from Russia with the Russian government's knowledge is not known to me at this point," he said.
"This is a substance that is known to us and does not exist widely. It is only in the hands of a very, very limited number of parties. And I don't want to say anything further than that," he told reporters.
"You take a substance like was used, which is an extremely dangerous substance, into another country, into a public place, where you know many others are going to be exposed...it's almost beyond comprehension that a state, an organized state would do something like that," Tillerson said.
Asked whether the apparent attack on a NATO member would trigger an allied response, Tillerson said: "It certainly will trigger a response. I'll leave it at that."
Tillerson told reporters that he's grown "extremely concerned" about Russia, noting that he spent most of his first year in office trying to solve problems and narrow differences with the Kremlin. He said after a year of trying, "we didn't get very far."
"Instead what we've seen is a pivot on their part to be more aggressive," Tillerson said. "And this is very, very concerning to me and others that there seems to be a certain unleashing of activity that we don't fully understand what the objective behind that is."