The United States slapped sanctions on Chechen strongman Ramzan Kadyrov on Wednesday, accusing Russian President Vladimir Putin's key regional ally of personal involvement in repression, torture and murder.

US officials accused Kadyrov of overseeing "an administration involved in disappearances and extrajudicial killings," and that "one or more" of his political opponents were killed at his direction.

Kadyrov was added to the US Treasury's blacklist along with Chechen security official Ayub Kataev and three Russians linked to the corruption case uncovered by deceased tax lawyer and whistleblower Sergei Magnitsky.

Kadyrov brushed off news of the sanctions, writing sarcastically on his Instagram social media account, "A sleepless night is waiting for me."

"I can be proud that I'm out of favor with the special services of the USA," he continued. "In fact, the USA cannot forgive me for dedicating my whole life to the fight against foreign terrorists among which there are bastards of America's special services."

"Treasury remains committed to holding accountable those involved in the Sergei Magnitsky affair, including those with a role in the criminal conspiracy and fraud scheme that he uncovered," Director of the Treasury Department's Office of Foreign Assets Control John Smith said in a statement.

The Magnitsky Act is a human rights law that was passed by the US Congress and has been a major source of tension between Washington and Moscow, even before Russia's annexation of Crimea in 2014 sent bilateral relations spiraling.

In response to the Magnitsky Act, Putin hit back, signing a bill that prohibited US adults from adopting Russian children.

State-sponsored-murder

The 37-year-old Magnitsky was arrested in 2008 after he uncovered a $230 million (€193 million) tax fraud scheme, according to US authorities. He died in prison nearly a year later. His death occurred little more than a week before he would have been released because his case had not come to trial.

Read more: Russian court convicts Magnitsky posthumously

Magnitsky's supporters accuse the Russian government of state-sponsored-murder, denying him adequate medical care after he was imprisoned on tax evasion charges. The Kremlin denies the allegation.

It had been unclear whether US President Donald Trump would continue to target people under the law, which currently has placed sanctions on 49 Russians.

The Magnitsky Act attracted renewed public attention after it emerged that the president's son Donald Trump Jr., had met with a Russian lawyer and a lobbyist — both strident opponents of the law — in New York ahead of the 2016 US elections.

When asked about the June 2016 meeting, Trump Jr. admitted that they discussed the adoptions issue.

The sanctions can freeze all assets an individual holds in the United States and also bars Americans from doing any business with them.

Last year the Magnitsky Act was expanded to allow the US to target human rights abusers anywhere, not just in Russia. The Trump administration is expected to use the law to target more individuals in the coming days.

bik/sms (AP, Reuters, AFP, dpa)