LONDON — The former Russian double agent got a terrifying message on his birthday: He was on a Kremlin hit list along with Sergei Skripal, another ex-spy who weeks later was poisoned with a nerve agent in a case Britain blames on Vladimir Putin's government.

"Be careful, look around, something is probably going to happen,'" the former agent, Boris Karpichkov, says an old friend told him on the telephone in mid-February. "It's very serious, and you are not alone."

Boris Karpichkov. NBC News

Among the names on the list was that of Skripal, whom Karpichkov didn't know at the time but whose poisoning alongside his daughter, Yulia, on March 4 on British soil inflamed tensions between the Kremlin and the West and triggered international condemnation. The two are in a hospital in Britain, where Skripal is in critical condition. Yulia is "improving rapidly" and is no longer in critical condition, the hospital treating the pair said Thursday.

Also on the Kremlin's list, he says, were several other ex-KGB agents, as well as Christopher Steele, author of a 35-page dossier alleging collusion between Russia and the Trump campaign. Bill Browder, the driving force behind a set of U.S. sanctions against Russian individuals known as the Magnitsky Act, was there as well, he adds.

Karpichkov, 59, says that at first he thought the call was a joke rather than a threat — typically dark Russian humor. But Skripal's poisoning has put him on high alert. “Trademark FSB,” he says, referring to Russia’s security agency, the Federal Security Service, the successor to the Soviet-era KGB. NBC News interviewed Karpichkov over the weekend at a rented studio in London; he refused to say where he lives in the U.K.

Putin has denied Russian involvement in the Skripal case, calling the allegations “nonsense.”

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What began as a spat between London and Moscow has snowballed into a chorus of international criticism of the Kremlin, with a series of governments ejecting dozens of Russian diplomats.

On Monday, the U.S. announced that it was expelling 60 Russians, which followed the U.K.'s decision to kick out 23. Russia’s foreign minister said on Thursday that Moscow would expel the same number of diplomats from each nation that has expelled Russian diplomats.