Western allies voiced support for Britain late Monday, after Prime Minister Theresa May said it was “highly likely” that Moscow was responsible for a nerve agent attack on a Russian former double agent.

In its first public comments on the case, the United States said it had “full confidence” in the British investigation and its assessment that Russia was probably behind the attempted assassination of Sergei Skripal and his daughter in southwest England on March 4.

Secretary of State Rex Tillerson said: “We agree that 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.”

Skripal and his daughter Yulia, 33, remain in a critical condition in hospital after being found unconscious on a bench outside a shopping centre in the sleepy city of Salisbury. In a dramatic escalation of diplomatic tensions over the incident, May said in parliament that the nerve agent used to poison the pair was a military-grade type developed by Russia.

She told British lawmakers that Moscow had previously used this group of nerve agents known as Novichok, had a history of state-sponsored assassinations and viewed defectors such as Skripal as legitimate targets.

“The government has concluded that it is highly likely that Russia was responsible for the act against Sergei and Yulia Skripal,” she said.

The prime minister added that Britain had given Moscow until the end of Tuesday to disclose details of its development of the Novichok nerve agents programme to the Organisation for the Prohibition of Chemical Weapons.

If there was “no credible response” it will conclude it was “an unlawful use of force by the Russian State against the UK”, she warned, and pledged to outline a “full range of measures” in response on Wednesday.