A close ally of Vladimir Putin has promised to make a “nice juicy steak” out of the opposition leader Alexei Navalny in a bizarre threat posted on the YouTube channel for Russia’s National Guard.

Viktor Zolotov, a former bodyguard to Putin who now heads Rosgvardiya, Russia’s internal military force, challenged Navalny to a duel during a six-minute rant in which he denied corruption allegations made by the opposition leader.

“Nobody has ever given you the spanking you deserve, so hard that you felt it in your liver,” the senior Russian official said in the video, which was posted online on Tuesday.

He went on to say that Navalny had made libellous statements against him in an investigation posted online and demanded “satisfaction”.

“I simply challenge you to a duel. In the ring, on the mat, wherever,” he continued. “I promise in several minutes to make a nice, juicy steak out of you.”

The video marked an unusual public appearance for Zolotov, who has never been an active politician and rose through the ranks of the Russian security services beginning in the 1970s. He has known Putin since both worked in St Petersburg in the 1990s and was widely seen as a trusted hand who could ensure the loyalty of Russia’s new National Guard, which he was tapped to head in 2016.

The Kremlin distanced itself from Zolotov’s words, saying he had not cleared them with the president, but did not censure the Russian general. “You know, sometimes any possible methods should be employed in opposing shameless libel,” Dmitri Peskov, Putin’s spokesman, told reporters on Tuesday. He said he did not see Zolotov’s remarks as a physical threat.

Russian newspapers have speculated about Zolotov’s personal wealth. Navalny last month published an investigation into alleged price inflation for tenders made by the National Guard.

Facebook Twitter Pinterest Viktor Zolotov with Putin last year. Photograph: Mikhail Klimentyev/AFP/Getty

Zolotov’s verbal assault came two days after Russian National Guard troops alongside riot police detained more than 1,000 protesters across Russia demonstrating against a proposed pension reform that would raise the retirement age for men and women by five years.

Navalny had called for the nationwide protests before he was jailed for 30 days late last month on a charge linked to earlier protests. Supporters have said he was detained to prevent him from leading Sunday’s rallies, in which police used batons against demonstrators, many of whom were in their teens and early 20s.

Zolotov’s motivation was not clear and commentators were at pains to assess why it was made. His statement also broke a Kremlin taboo by mentioning Navalny by name. Putin has avoided doing so on camera.

Navalny could not immediately be reached for comment on Zolotov’s video because he was in jail.

Other Putin allies have released similar video attacks against Navalny. In 2017, the Russian tycoon and former Arsenal shareholder Alisher Usmanov accused Navalny of slander in a YouTube video and demanded he apologise for another investigation.