TALLINN, Estonia (AP) — The whereabouts of imprisoned Russian opposition leader Alexei Navalny remained unknown on Friday, with penitentiary officials reporting that earlier this month he was moved from the region where he was serving time, but still not disclosing where he is, the politician's allies said.

Navalny's lawyers haven't seen him since Dec. 6, his spokesperson Kira Yarmysh said on X, formerly known as Twitter.

Navalny has been serving a 19-year term on charges of extremism in a maximum-security prison, Penal Colony No. 6, in the town of Melekhovo in the Vladimir region, about 230 kilometers (140 miles) east of Moscow. He was due to be transferred to a “special security” penal colony, a facility with the highest security level in the Russian penitentiary system.

Russian prison transfers are notorious for taking a long time, sometimes weeks, during which there’s no access to prisoners and information about their whereabouts is limited or unavailable. Navalny could be transferred to one of a number of such penal colonies across Russia.

Navalny's lawyer was informed at a court hearing Friday that his client “left the Vladimir region” on Dec. 11, Yarmysh said in a tweet. “Where exactly (he was moved to) — unclear,” she wrote.

Vyacheslav Gimadi, head of the legal department at Navalny's Anti-Corruption Foundation, said on X that the information came from a penitentiary service statement that was read out in court.

Navalny, 47, has been behind bars since January 2021. As President Vladimir Putin’s fiercest foe, he campaigned against official corruption and organized major anti-Kremlin protests. His arrest came upon his return to Moscow from Germany, where he recuperated from nerve agent poisoning that he blamed on the Kremlin.

Navalny has since been handed three prison terms and spent months in isolation in the penal colony in the Vladimir region for alleged minor infractions.

He has rejected all charges against him as politically motivated.

His allies sounded the alarm last week, saying that Navalny's lawyers were not let into the penal colony to see him, letters to the politician were not being delivered and he was not appearing at scheduled court hearings via video link.

Yarmysh said last Friday that those developments were concerning given that Navalny recently fell ill and apparently fainted “out of hunger.” She said he is being "deprived of food, kept in a cell without ventilation and has been offered minimal outdoor time.”