Britain blames Russia for the poisoning of ex-spy Sergei Skripal and his daughter Yulia in England last month, triggering mutual diplomatic expulsions. Skripal is one of four Russians convicted of betraying Moscow for the West who were exchanged for 10 Russian agents in 2010.

The United States has warned that former Russian double agents who were swapped to the West for Russian spies could face danger in the wake of the poisoning of an ex-agent in Britain.

“We have massive concerns about any Russian critic or other who is now here,” The Daily Telegraph quoted a senior U.S. official as saying late on Sunday.

The official stressed that the Skripals’ poisoning, which Russia denies and accuses Britain of masterminding to stoke anti-Russian hysteria, “was a definite rupture of the rules of the game.”

“When you do a spy swap you don’t expect them to be bumped off later,” the anonymous official was cited as saying.

The official added that the Skripals’ poisoning produced “a chilling effect for anyone who might be contemplating” cooperating with foreign intelligence.

Reports on Sunday suggested that London was considering to offer the Skripals resettlement in the U.S. in an attempt to protect them from possible murder attempts.