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, putting him at odds with the President who has so far stayed silent on the issue.
Mr Tillerson's comments amount to the strongest US response yet to Theresa May's declaration that it was "highly likely" Russia was behind the horrific poisoning in Salisbury on March 4.
Unlike Mr Tillerson, the White House has only condemned the attack as "reckless, indiscriminate and irresponsible", with no mention made of Russia, or its President, Vladimir Putin.
In a statement, Sarah Huckabee Sanders, a spokesperson for the President, simply said: "The use of a highly lethal nerve agent against UK citizens on UK soil is an outrage.
"We offer the fullest condemnation and we extend our sympathy to the victims and their families and our support to the UK Government.
"We stand by our closest ally and the special relationship that we have."
President Trump's silence on any links to Russia puts him at odds with his Secretary of State who said that the Novichok nerve agent was "only in the hands of a very, very limited number of parties" and that it "clearly came from Russia".
However, Mr Tillerson has stated that he does 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.
After speaking to Foreign Secretary Boris Johnson, Mr Tillerson 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."
The Kremlin has denied any involvement in the nerve agent attack on Mr Skripal, a former Russian intelligence officer who was jailed as a double agent before being sent to the UK in a 2010 spy swap.
On Monday, 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 on Tuesday 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 on Tuesday morning to discuss the latest developments.
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.
The attack on Mr Skripal and his daughter has also been condemned by Nato, with Secretary General Jens Stoltenberg branding "the use of any nerve agent horrendous and completely unacceptable.
"The UK is a highly valued ally, and this incident is of great concern to Nato. Nato is in touch with the UK authorities on this issue."
Mrs May's dramatic statement to the Commons on Monday came after Mr 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.
Following Mrs May's statement, news agency Tass quoted Russian Foreign Ministry spokesperson Maria Zakharova as saying: "It is a circus show in the British Parliament.
"The conclusion is obvious, it's another political information campaign, based on a provocation."
And Mr Putin dismissed questions about the Skripals when he was confronted during an election campaign visit, telling the BBC: "Get to the bottom of things there, then we'll discuss this."
The National Security Council is expected to meet on Wednesday to discuss the Russian response, if any, ahead of a statement by the Prime Minister.