Russia's Foreign Minister Sergei Lavrov says Moscow will not respond to Britain's ultimatum until it is given access to the substance used to poison a former spy.
Russian Foreign Minister Sergei Lavrov on Tuesday insisted that Moscow is not to blame for the poisoning of Russian former double agent Sergei Skripal in Britain and said it was ready to cooperate with London.
"Russia is not guilty. Russia is ready to cooperate according to the Chemical Weapons Convention, if Britain takes the trouble and condescends to carry out its international obligations according to the same document," Lavrov said at a press conference.
Moscow faces a midnight Tuesday deadline to tell London how a Russian-made nerve agent came to be used in the brazen poisoning of Mr Skripal, with Prime Minister Theresa May threatening "a full range of measures" in retaliation.
The US, NATO and the European Union have all backed Britain in the deepening diplomatic row.
Foreign Secretary Boris Johnson was encouraged by the support of "friends", describing the attack as the "first use of nerve agent on the continent of Europe since the end of the Second World War."
He vowed that Britain's response would be "commensurate but robust", if it concludes Russia was responsible.
British ambassabdor to Russia summoned
The Russian foreign ministry said on Tuesday it had summoned the British ambassador amid the escalating row over the poisoning of the former double agent.
A foreign ministry spokeswoman, Maria Zakharova, said the ambassador, Laurie Bristow, was summoned but did not provide further details.
May has said that her government was considering a British version of the US "Magnitsky Act", which was adopted in 2012 to punish Russian officials accused of human rights violations.
Skripal, 66, 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 southwestern city of Salisbury.
Emergency workers in biohazard suits have been deployed in the normally sleepy city, while about 500 people who may have come into minimal contact with the nerve agent were urged to wash clothes and belongings as a precaution.
'They will not recover'
May told British lawmakers that Moscow had previously used a group of nerve agents known as Novichok, had a history of state-sponsored assassinations and viewed defectors such as Skripal as legitimate targets.
She demanded Moscow disclose details of its development of the Novichok nerve agents programme to the Organisation for the Prohibition of Chemical Weapons.
Vil Mirzayanov, a chemist who worked on the Novichok programme and now lives in the United States, was quoted as saying that the nerve agent's effects were "brutal".
"These people are gone -- the man and his daughter. Even if they survive they will not recover," he was quoted as saying.
US Secretary of State Rex Tillerson said Washington has "full confidence" in the British investigation, adding that it was "almost beyond comprehension" that a state would use such a dangerous substance on public streets.