The nerve agent attack on ex-spy Sergei Skripal and his daughter Yulia "clearly came from Russia" and "certainly will trigger a response", US Secretary of State Rex Tillerson has said.
Tillerson's comments amount to the strongest US response yet to Theresa May's declaration that it was "highly likely" Russia was behind the poisoning in Salisbury on March 4.
The Prime Minister told MPs that the highly dangerous substance used in the attack was a military-grade Novichok nerve agent produced by Russia.
She set a deadline of midnight tonight (March 13) for Moscow to explain whether it was behind the attack or had lost control of its stockpile of the poison.
Failure to provide a "credible" response would lead her to view the incident as "an unlawful use of force by the Russian State against the United Kingdom," sparking unspecified measures in reprisal.
Home Secretary Amber Rudd will chair a meeting of the Government's Cobra emergencies committee in Whitehall this morning (March 13) to discuss the latest developments.
According to AP, Tillerson told journalists travelling with him in Africa that the Novichok agent was "only in the hands of a very, very limited number of parties."
Although he said it "clearly came from Russia," he added that he did not know whether Vladimir Putin's government had knowledge of the poisoning and said it was "almost beyond comprehension" that a state actor would use such a dangerous substance in a public place.
In a formal statement released after a phone call with Foreign Secretary Boris Johnson, the US Secretary of State said: "We have full confidence in the UK's investigation and its assessment that Russia was likely responsible for the nerve agent attack that took place in Salisbury last week.
"Those responsible - both those who committed the crime and those who ordered it - must face appropriately serious consequences. We stand in solidarity with our Allies in the United Kingdom and will continue to co-ordinate closely our responses."
With the world weighing up the possibility of sanctions against Russia, French president Emmanuel Macron offered his country's solidarity with the UK in a phone call with Mrs May, in which he said that Paris would "co-ordinate closely" with London following Russia's response.
May's dramatic statement to the Commons on Monday came after Johnson summoned Russian ambassador Alexander Yakovenko to the Foreign Office to voice Britain's outrage, giving him little more than 24 hours to provide Moscow's response.
The Kremlin has denied any involvement in the nerve agent attack on Skripal, a former Russian intelligence officer who was jailed as a double agent before being sent to the West in a 2010 spy swap.