US sanctions 4 Russians allegedly involved in poisoning of Alexey Navalny

The U.S. on Thursday sanctioned four Russians they say were involved in the 2020 poisoning of Alexey Navalny, the country's most well-known opposition leader.

The Treasury Department said the four individuals were members of Russia's Federal Security Service, or the FSB -- the main successor agency to the Soviet Union's KGB.

"Today we remind Vladimir Putin and his regime that there are consequences not only for waging a brutal and unprovoked war against Ukraine, but also for violating the human rights of the Russian people," Brian Nelson, the under secretary of the Treasury for Terrorism and Financial Intelligence, said in a statement.

"The assassination attempt against Aleksey Navalny in 2020 represents the Kremlin's contempt for human rights, and we will continue to use the authorities at our disposal to hold the Kremlin's willing would-be executioners to account," Nelson added.

Navalny, one of Russian President Vladimir Putin's fiercest critics, was poisoned with a Novichok nerve agent while on a plane to Moscow in August 2020. He was flown to Germany, where he received treatment and doctors confirmed he'd been poisoned.

Navalny has been in prison since he returned to Moscow in 2021 after his recovery. Earlier this year, he was sentenced to 19 more years in jail on extremism charges. The U.S. has called the charges "unfounded" and called for his immediate release.

The four Russians targeted by the Treasury Department -- Alexey Alexandrovich Alexandrov, Konstantin Kudryavtsev, Ivan Vladimirovich Osipov and Vladimir Alexandrovich Panyaev -- had previously been sanctioned by the U.S., but were newly sanctioned Thursday under the Magnitsky Act, which allows the U.S. government to target people involved in extrajudicial killings and other human rights abuses.

According to the Treasury Department, the men "collaborated to surveil Navalny ahead of the attack, break into his hotel room and apply the chemical weapon to his personal belongings, and they attempted to erase any evidence of their operation following the attack."

The U.S. State Department on Thursday also imposed visa restrictions on the four men.

Navalny said in December 2020 he'd tricked a Russian intelligence officer, identified as Kudryavtsev, into accidentally admitting the role he and others played in the assassination attempt. The 49-minute conversation was posted online.

The FSB said the call was "fake," and Putin has long denied Russia was involved in Navalny's poisoning.

-ABC News' Shannon Crawford contributed to this report.