LONDON • Russia has been researching the application of chemical agents to door handles as a way to assassinate its enemies and has been training personnel "from special units" to carry out such attacks, said Mr Mark Sedwill, Britain's national security adviser.

Mr Sedwill sent a letter to the secretary-general of Nato on Friday - the most detailed account of British intelligence on the subject to be shared with the public to date.

It also revealed that President Vladimir Putin of Russia was "closely involved in the chemical weapons programme" beginning in the mid-2000s.

During that period, the letter claims, Russia was secretly developing the nerve agent known as Novichok that British officials say was used in the March 4 attack on former Russian double agent Sergei Skripal and his daughter, Yulia, in the quiet cathedral city of Salisbury, England.

Russian officials have strenuously denied producing Novichok or carrying out the attack, which has brought relations between Britain and Russia to a post-Cold War low.

In a news conference on Friday, the Russian ambassador to Britain, Mr Aleksandr Yakovenko, dismissed the letter and "all these allegations" surrounding the nerve agent attack as having "nothing to do with reality".

Russia, he said, repeating a claim the Kremlin has asserted throughout the Skripal affair, eliminated all of its stockpiles of chemical weapons last year, and as for Novichok, "we did not produce it and didn't store it". Mr Skripal remains hospitalised nearly five weeks after he was poisoned, but his daughter has recovered and was moved to a secure location last week.

Mr Sedwill's letter also states that Britain has evidence that Russian security services have been monitoring the Skripal family.

Cyber specialists from Russia's Foreign Intelligence Services hacked Ms Yulia Skripal's e-mails in 2013, the letter says.

Asked about that at his news conference, Mr Yakovenko responded sarcastically: "Big surprise."

The letter adds that Russian intelligence services "view at least some of its defectors as legitimate targets for assassination".

"We therefore continue to judge that only Russia has the technical means, operational experience and motive for the attack on the Skripals and that it is highly likely that the Russian state was responsible," the letter says. "There is no plausible alternative explanation."

The letter comes as British officials try to consolidate European support for united actions against Russia. The central element of Britain's case against Russia is the unusual nerve agent used in the attack, which was developed in Soviet laboratories during the last years of the Soviet Union.

Mr Sedwill's letter lays out further British intelligence on Russia's chemical weapons programmes, reporting that the Novichok agents - a strain referred to in Russia as Foliant - were developed at the State Institute for Organic Chemistry and Technology at Shikhany, a small town on the Volga river, in southern Russia.

It says that Russia continued to produce the agents after the breakup of the Soviet Union, but did not declare the work to the Organisation for the Prohibition of Chemical Weapons.

It goes on to say that "during the 2000s", Russia created a special unit to develop chemical weapons for use as tools in state-sponsored attacks and to "train personnel from special units in the use of these weapons".

"This programme subsequently included investigation of ways of delivering nerve agents, including by application to door handles," it says.

NYTIMES