British Prime Minister Theresa May has said it was "highly likely" that Moscow was responsible for the poisoning in England of a former Russian spy Sergei Skripal and his daughter Yulia.
Either the Russian state was directly responsible for the poisoning or it had allowed a nerve agent to get into hands of others, Mrs May told Britain's parliament.
"It's highly likely that Russia was responsible for the act against Sergei and Yulia Skripal." pic.twitter.com/RIxioHzthv— RTÉ News (@rtenews) March 12, 2018
Mrs May said Russia's ambassador to London had been summoned to explain to British Foreign Secretary Boris Johnson how the nerve agent came to have been used in the 4 March incident in the English cathedral city of Salisbury.
British officials had identified the substance as being part of the Novichok group of nerve agents which were developed by the Soviet military during the 1970s and 1980s, Mrs May said.
"Should there be no credible response, we will conclude that this action amounts to an unlawful use of force by the Russian state against the United Kingdom," she said, calling the attack a "reckless and despicable act".
Russia's foreign ministry hit back immediately, saying Mrs May's comments were a "circus show" and part of a political information campaign against Russia.
White House spokeswoman Sarah Sanders said US President Donald Trump's administration stood by America's "closest ally".
"The use of a highly lethal nerve agent against UK citizens on UK soil is an outrage," Ms Sanders said.
"The attack was reckless, indiscriminate and irresponsible. We offer the fullest condemnation."
US secretary of State Rex Tillerson also said Russia was "likely responsible" for the attack.
Mr Tillerson added: "We agree that those responsible - both those who committed the crime and those who ordered it - must face appropriately serious consequences."
Mr Skripal, 66, and his 33-year-old daughter Yulia, have been in hospital in a critical condition since being found unconscious on a bench outside a shopping centre in the city.
A British policeman who was one of the first to attend to the stricken spy was also affected by the nerve agent.
He is now conscious in a serious but stable condition, police said.
"On Wednesday we will consider in detail the response from the Russian state," Mrs May said.
"Should there be no credible response, we will conclude that this action amounts to an unlawful use of force by the Russian state against the United Kingdom."
Mrs May said the poisoning took place "against a backdrop of a well-established pattern of Russian state aggression" and she said Britain was ready to take "much more extensive measures" against Russia than in the past.
Russia's foreign ministry hit back immediately, saying the comments by Mrs May were a "circus show" and part of a political information campaign against Russia, local media reported.
Earlier today, before Mrs May spoke, Russian President Vladimir Putin brushed off a question about the affair, saying British authorities should first "get to the bottom of things," the BBC's Moscow correspondent wrote on Twitter.
Mrs May's speech came after she chaired a meeting of Britain's National Security Council (NSC).
It met to discuss how to respond to the nerve agent attack on Mr Skripal, who passed secrets to British intelligence and was found collapsed with his daughter.
"I think the cabinet ... is very clear that this is an ongoing investigation, that it is important that we allow the police to get on with their work," the spokesman told reporters.
"If we get to a position when we're able to attribute this attack then we will do so and the government will deliver an appropriate response."
The NSC brings together senior ministers from relevant departments and is attended by intelligence and military chiefs as required.
Measures potentially on the table if they do decide to take action could include the expulsion of Russian diplomats and spies, new financial sanctions against individuals linked to the Kremlin, and withdrawal of official representation from the World Cup in Russia.
Meanwhile, Kremlin spokesman Dmitry Peskov said that the poisoning of Mr Skripal was not a matter for the Russian government.
"The mentioned Russian citizen worked for one of the British intelligence services, the incident occurred in Great Britain. This is not a matter for the Russian government," he said.
Mr Peskov also said he had not heard allegations from British politicians about possible Russian involvement in the attack.
Yesterday, hundreds of diners and pub-goers were urged to wash clothes and other items a week after potentially coming into contact with the nerve agent.
The "precautionary advice" was issued after traces of the substance were found in The Mill pub and the nearby Zizzi restaurant in Salisbury.
In other developments Wiltshire Police told residents "not to be alarmed" as counter-terror police were helped by armed forces, including royal marines, in the clean-up operation.
Eight marked and unmarked police cars were removed from Bourne Hill station in the biggest military operation yet.
Two fire and rescue lorries were seen entering the police cordon leading to Mr Skripal's house, while investigators in hazmat suits bagged up items inside The Mill pub.
England's chief medical officer Dame Sally Davies said she believed up to 500 people would have been in either of the two venues between the Sunday lunchtime and Monday night.
She said: "I want to reassure the general public that the risk to us all from this incident in Salisbury has not changed, and that the risk to us all remains low."
The advice included machine wash clothing worn on the day and double-bagging items that would normally be dry cleaned to await further advice.