LONDON (AP) — In a letter made public Friday, British National Security Adviser Mark Sedwill said that Russian intelligence agencies have been spying on Sergei and Yulia Skripal for at least five years.

Sedwill made the assertion in a letter to NATO Secretary General Jens Stoltenberg explaining Britain's conclusion that the Russian government is to blame for the military-grade nerve agent used against the Skripals in a March 4 attack.

He said only Russia has the "technical means, operational experience and the motive" for the attack against the former Russian spy and his daughter.

In the letter, Sedwill said the Soviet Union developed fourth generation nerve agents known as Novichoks in the 1980s at the State Institute for Organic Chemistry and Technology near Volgograd under the codeword FOLIANT.

"It is highly likely that Novichoks were developed to prevent detection by the West and to circumvent international chemical weapons controls," he said. "The Russian state has previously produced Novichoks and would still be capable of doing so."

He said that after the Soviet Union collapsed, Russia signed the Chemical Weapons Convention without reporting its ongoing work on Novichoks.

"Russia further developed some Novichoks after ratifying the convention," he said, adding that it was highly unlikely that any former Soviet republic besides Russia pursed an offensive chemical weapons program.

He said Moscow had a proven record of state-sponsored assassinations and had tested ways of delivering chemical weapons, including the use of door handles to spread nerve agents, as Britain believes was done in the Skripal case.

He also said Russia had a clear motive for attacking Skripal, a former Russian military intelligence officer who had been imprisoned in Russia for spying for Britain only to be set free in a spy swap.

"It is highly likely that the Russian intelligence services view at least some of its defectors as legitimate targets for assassination," he said. "We have information indicating Russian intelligence service interest in the Skripals, dating back at least as far as 2013, when e-mail accounts belonging to Yulia Skripal were targeted by GRU (military intelligence) cyber specialists."

Russia's ambassador to the United Kingdom, Alexander Yakovenko, dismissed the charges Friday as unfounded and untrue.

Yulia Skripal, 33, was released from the hospital this week. Her father remains hospitalized but British health officials say he is improving.

Russia has charged that British officials were keeping Skripal's daughter in isolation. It demanded access to her and prodded Britain to share evidence in the case.

Russia strongly denies the British claims about Novichok, saying that it completed the destruction of all its Soviet-era chemical weapons arsenals last year under international oversight. It insisted that the nerve agent used on the Skripals could easily have been manufactured in any of the other countries that have advanced chemical research programs.

Russia's Foreign Minister Sergey Lavrov said Friday that a new report on the poisoning of a former spy does nothing to support Britain's contention that Russia was behind the attack.

Investigators at the Organization for the Prohibition of Chemical Weapons confirmed British findings that the Skripals were poisoned with a military-grade nerve agent. But it didn't say who was responsible for the poisoning.

Lavrov said the report does nothing to back the British allegations that Moscow was behind the attack, which Moscow denies.

"I want to underline: the OPCW only has confirmed the composition of the chemical agent," Lavrov said at a news conference.

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Isachenkov reported from Moscow.