The British military facility analysing the nerve agent used to poison a spy said on Tuesday it could not prove the substance was made in Russia, as Russian President Vladimir Putin put his hopes in a meeting with the world's chemical weapons watchdog.
Gary Aitkenhead, chief executive of the Porton Down defence laboratory, told Britain's Sky News that analysts had identified the substance as military-grade Novichok, the word used for a category of nerve agents developed in Soviet times.
But he added: "We have not identified the precise source."
"It is our job to provide the scientific evidence of what this particular nerve agent is, we identified that it is from this particular family and that it is a military grade, but it is not our job to say where it was manufactured," Aitkenhead said.
He added that "extremely sophisticated methods" were needed to create the nerve agent, and that was "something only in the capabilities of a state actor"
Following his remarks, a British government spokesman said Porton Down's identification of Novichok was "only one part of the intelligence picture".
"This includes our knowledge that within the last decade, Russia has investigated ways of delivering nerve agents probably for assassination - and as part of this programme has produced and stockpiled small quantities of Novichok; Russia's record of conducting state-sponsored assassinations; and our assessment that Russia views former intelligence officers as targets," he added.
Former double agent Sergei Skripal, who has lived in Britain since a spy swap in 2010, and his daughter Yulia have been in hospital since March 4 after the poisoning that London and its major Western allies have blamed on Russia.
The first use of chemical weapons in Europe since World War II has chilled Moscow's relations with the West, as both sides have expelled scores of diplomats.
Britain has also suspended high-level diplomatic contact with Moscow.
Moscow has denied any involvement in the incident, with Russian Foreign Minister Sergei Lavrov on Monday suggesting that the British government may be behind the poisoning to distract attention from problems around Brexit.
Aitkenhead would not comment on whether Porton Down had developed or keeps stocks of Novichok, but he dismissed claims the agent used to poison the Skripals had come from the site.
"There is no way anything like that could have come from us or left the four walls of our facility," he said.
The Organisation for the Prohibition of Chemical Weapons (OPCW), the world's chemical watchdog, will meet Wednesday to discuss Britain's allegations that Russia was responsible for the poisoning, according to documents released on Tuesday.
The OPCW said Russia had asked for the meeting, which Putin said was aimed at putting an end to the ongoing dispute.
"We hope that during those discussions a full stop will be placed on (the issue of) what happened," he said during a visit to the Turkish capital Ankara.
The meeting request came after Moscow received and analysed samples of the Novichok agent used in the attack.
The confidential gathering will start at 10:00 am (0800 GMT) at the OPCW's headquarters in The Hague.
"We hope to discuss the whole matter and call on Britain to provide every possible element of evidence they might have in their hands," Russia's ambassador to Ireland, Yury Filatov, told reporters ahead of the OPCW meeting.
"Russia is interested in establishing the whole truth of the matter," he said.
But Britain's foreign ministry accused Russia of requesting the meeting to undermine the organisation's investigation.
"This Russian initiative is yet again another diversionary tactic, intended to undermine the work of the OPCW in reaching a conclusion," the ministry said in a statement.
"Of course, there is no requirement in the Chemical Weapons Convention for the victim of a chemical weapons attack to engage in a joint investigation with the likely perpetrator," it said.