The US has thrown its diplomatic weight behind the UK as Britain braces itself for Russian retaliation after Theresa May's clampdown yesterday.
The Kremlin said it is readying its response to May's removal of 23 Russian diplomats, the biggest expulsion of the country's embassy staff since the Cold War, in response to the poisoning of Sergei Skripal.
The White House has come down firmly on Britain's side, saying the White House "stands in solidarity with its closest ally, the United Kingdom.
"The United States shares the United Kingdom's assessment that Russia is responsible for the reckless nerve agent attack on a British citizen and his daughter, and we support the United Kingdom's decision to expel Russian diplomats as a just response.
"This latest action by Russia fits into a pattern of behaviour in which Russia disregards the international rules-based order, undermines the sovereignty and security of countries worldwide, and attempts to subvert and discredit Western democratic institutions and processes."
The comments came after the diplomatic drama shifted to the UN Security Council.
A showdown gathering at the world body saw Britain call on the international chemical weapons watchdog to verify its findings that Moscow is behind the Salisbury incident.
The UK's deputy UN ambassador, Jonathan Allen, told a special meeting of the Security Council that the Organisation for the Prohibition of Chemical Weapons had been asked to go over the British analysis of the attack.
In heated exchanges at the Security Council gathering, Russia strongly denied it was involved in the Salisbury incident, and the US offered Britain its full support.
US ambassador to the UN Nikki Haley said: "The United States believes that Russia is responsible for the attack on two people in the United Kingdom using a military-grade nerve agent. Dozens of civilians and first responders were also exposed.
"If we don't take immediate concrete measures to address this now, Salisbury will not be the last place we see chemical weapons used. This is a defining moment."
The Russian permanent representative to the UN, Vassily Nebenzia, said: "We demand that material proof be provided of the allegedly found Russian trace in this high-resonance event.
"Without this, stating that there is incontrovertible truth is not something that we can take into account."
Yesterday (March 14) French officials declined to directly blame Russia until the country's involvement was proven.
However a subsequent statement by the French Embassy said the country's "solidarity with the UK is unquestionable."