Britain's military research centre has acknowledged it is unable to say whether the military-grade nerve agent used to poison a Russian double-agent last month was produced in Russia.

Key points: Military research chief says only a state actor could have created agent

Military research chief says only a state actor could have created agent British Government stands by its accusations against Russia

British Government stands by its accusations against Russia Vladimir Putin says anti-Russia campaign "causes bewilderment"

The Kremlin has seized on the statement, saying it proves that British accusations of Moscow's involvement are baseless.

Scientists at the Defence Science and Technology Laboratory at Porton Down previously identified the poison as a Soviet-developed type of nerve agent known as Novichok.

The British Government has said the only plausible explanation was that it came from Russia and blamed Russia for the attack on Sergei Skripal and his daughter, Yulia.

"We were able to identify it as Novichok, to identify that it was military-grade nerve agent," Porton Down chief executive Gary Aitkenhead told Sky News.

"We have not identified the precise source, but we have provided the scientific info to Government who have then used a number of other sources to piece together the conclusions you have come to."

However, he confirmed the substance required "extremely sophisticated methods to create, probably something only in the capabilities of a state actor".

He added: "We are continuing to work to help to provide additional information that might help us get closer to [the source] but we haven't yet been able to do that."

Russian officials have suggested the poison could have come from Britain, pointing out that Porton Down conducts secret chemical and biological weapons research.

Sorry, this video has expired UK police says Sergei Skripal and his daughter were likely poisoned at the font door of their home (Photo: AP/Frank Augstein)

Russian Deputy Foreign Minister Alexander Grushko called the poisonings a "provocation arranged by Britain" to justify high military spending because "they need a major enemy".

Mr Aitkenhead said there was "no way" the nerve agent could have come from the high-security facility.

"We deal with a number of very toxic substances as part of the work that we do," he said.

"We've got the highest levels of security and controls," he said.

Vladimir Putin hits back at Britain

Russian President Vladimir Putin quickly pointed at Mr Aitkenhead's statement as evidence that British accusations of Russian involvement were unfounded.

Moscow has fiercely denied being behind the March 4 attack in the southern English city of Salisbury.

"The speed at which the anti-Russian campaign was launched causes bewilderment," Mr Putin said from Turkey, where he met with President Recep Tayyip Erdogan.

After the first known use of a military-grade nerve agent on European soil since World War II, Britain blamed Mr Putin for the attempted murder, and the West has expelled about 130 Russian diplomats.

Armin Laschet, a conservative ally of German Chancellor Angela Merkel, said the comment raised questions about Britain's drive to persuade allies to expel Russian diplomats.

"If one forces nearly all NATO countries into solidarity, shouldn't one have certain evidence?" Mr Laschet, premier of the state of North Rhine-Westphlia, said on Twitter.

"Regardless of what one thinks about Russia, my study of international law taught me a different way to deal with other states."

Sergei Skripal (L) and Yulia Skripal (R) are both still in hospital. ( AP: Misha Japaridze (L) Facebook: Yulia Skripal (R) )

A British government spokesperson said on Tuesday: "We have been clear from the very beginning that our world-leading experts at Porton Down identified the substance used in Salisbury as a Novichok, a military-grade nerve agent.

"This is only one part of the intelligence picture.

"As the Prime Minister has set out … 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 program has produced and stockpiled small quantities of Novichoks; Russia's record of conducting state-sponsored assassinations; and our assessment that Russia views former intelligence officers as targets."

Mr Skripal's daughter Yulia is getting better after spending three weeks in a critical condition, the hospital where she is being treated said last Thursday.

Her father remained in a critical but stable condition.

Reuters/AP