Imprisoned Russian opposition leader Aleksei Navalny has been placed in punitive solitary confinement for the fourth time since mid-August.

Navalny wrote on Instagram on September 7 that he was sent back to solitary confinement right after he finished his previous multiday term there. He did not say why he was returned to the punitive cell that in Russia's penitentiary system is a tiny concrete room with no toilet or running water.

"They additionally designated me as 'a persistent violator' [of internal regulations]. That means I will be put in more restrictive conditions in the penal colony of maximum security," Navalny wrote, adding that most likely he will be deprived of an upcoming three-day meeting with his wife and parents that he has waited four months for.

Two days earlier, a court in Kovrov in the Vladimir region said it registered a lawsuit filed by Navalny against the warden, Yury Korobov, of Penal Colony No. 6, about 260 kilometers east of Moscow.

Navalny claims in the lawsuit that he was unnecessarily placed in punitive confinement for a partially unbuttoned prison suit.

His other incarcerations in a punitive cell were for failing to carry out a guard's command to put his hand behind his back in a timely manner, and for "wrongly identifying himself" to a guard.

Navalny insists that he was placed in solitary confinement for political reasons, namely because of the activities that his associates continue from abroad and because he established a labor union in the prison.

Navalny was arrested in January 2021 upon his return to Moscow from Germany, where he was treated for a poison attack in Siberia in 2020 with what European labs defined as a Soviet-style nerve agent.

Navalny has blamed President Vladimir Putin for the poisoning, which the Kremlin has denied.

The corruption crusader was then handed a 2 1/2-year prison sentence for violating the terms of an earlier parole during his convalescence abroad. The original conviction is widely regarded as a trumped-up, politically motivated case.

In March, Navalny was sentenced in a separate case to nine years in prison on embezzlement and contempt charges that he and his supporters have repeatedly rejected as politically motivated.