Russia will refuse to meet Theresa May's midnight deadline unless Britain agrees to send Moscow samples of the nerve agent used to poison Sergei Skripal.
The country's embassy in the UK fired off a salvo of seven tweets in which it said Britain must comply with the Chemical Weapons Convention for a joint investigation and warned the threat of sanctions would "meet with a response".
It comes as President Donald Trump told Mrs May in a phone call the US is "with the UK all the way", according to Downing Street.
The Prime Minister had set a deadline of midnight on Tuesday for Moscow to explain whether it was behind the attack on ex-spy Sergei Skripal and his daughter Yulia or had lost control of its stockpile of the deadly nerve agent.
Home Secretary Amber Rudd has chaired a meeting of the Government's Cobra emergencies committee in Whitehall to discuss the latest developments.
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 coordinate 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 "coordinate closely" with London following Russia's response.
A Downing Street spokesman said: "They discussed the wide pattern of aggressive Russian behaviour and agreed that it would be important to continue to act in concert with allies to address it."
NATO secretary general Jens Stoltenberg said:
"The use of any nerve agent is 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.
US intelligence sources suspect as many as 14 people may have been assassinated on British soil by Russia's security services or mafia groups. Police and MI5 are to look into the string of deaths.
In each of the cases, police found no evidence of foul play, including the deaths of prominent Putin critic Boris Berezovsky and whistle-blower Alexander Perepilichnyy.
Former police chief Lord Blair, who led the Metropolitan Police at the time of the poisoning murder of Alexander Litvinenko in 2006 has called for the 14 cases to be reopened, while the cross-party House of Commons Home Affairs Committee has called for a review of decisions made by police.
In a letter to the Committee, written on Saturday and released on Tuesday, Ms Rudd said:
"The Government was aware of these allegations and takes seriously any suggestion that a foreign state has engaged in murder on UK soil."
"In the weeks to come, I will want to satisfy myself that the allegations are nothing more than that.
"The police and MI5 agree and will assist in that endeavour."
Russia is now asking for access to samples of the nerve agent Novichok, that poisoned ex-spy Sergei Skripal and his daughter Yulia in Salisbury, according to Foreign Minister Sergey Lavrov.
Mr Lavrov said Russia "is not to blame" for the incident on March 4.
The Russian Foreign Ministry has summoned the British ambassador in Moscow over the case, Russian news agencies have said.
Mr Lavrov warned that Russia will only co-operate with Britain in the investigation if it receives samples of the nerve agent that is believed to have been used to target Mr Skripal and his daughter.
He said that Moscow's requests to see samples of the nerve agent had been turned down, which he called a violation of the Chemical Weapons Convention, which outlaws the production of chemical weapons.
Moscow is willing to co-operate with the probe says Mr Lavrov, but he suggested London would be "better off" complying with its international obligations "before putting forward ultimatums".