The United States on Monday blacklisted Russian President Vladimir Putin's reputed top enforcer and two more Russians alleged to have fatally poisoned ex-spy Alexander Litvinenko in London a decade ago.

The Treasury added the prosecutor and senior investigator Alexander Bastrykin and alleged assassins Andrei Lugovoi and Dmitri Kovtun to the Magnitsky Act sanctions list.

Announcing the decision, the State Department did not elaborate on the new targets or the timing, which comes at a time of heightened tension with Moscow.

President Barack Obama's outgoing administration has accused the Kremlin of using cyber espionage, leaking and propaganda in a bid to influence November's White House race.

Moscow has scornfully rejected the charges, over which Washington has already expelled 35 Russian diplomats allegedly involved in espionage.

Monday's decision zeroes in on a close Putin ally.

Bastrykin is one of the Russian president's most powerful allies and as head of an investigative agency that had led crack downs on domestic dissidents and targeted foreign NGOs and other groups accused of meddling in Russian politics.

British officials have identified Lugovoi and Kovtun as main suspects in the death of Litvinenko, who succumbed to radiation poisoning in London in 2006 after drinking polonium-laced tea

The Magnitsky Act was originally passed to enable US officials to impose sanctions on Russians implicated in the 2009 prison death of Russian tax fraud whistleblower Sergei Magnitsky.

But more individuals have been blacklisted over the years.

The list now includes 44 names of those whose assets under US jurisdiction are frozen, and who are barred from doing business with Americans or receiving US visas.

The act allows for the designation of those implicated in the murder, torture or persecution of those who reveal corruption in Russia, State Department spokesman John Kirby said.

The US authorities also added two more less well-known Russian officials, Stanislav Gordievsky and Gennady Plaksin, to the list.

Last Update: Wednesday, 20 May 2020 KSA 09:48 - GMT 06:48