Jailed Russian opposition leader Alexei Navalny said he was "fine" Tuesday after a "pretty exhausting" 20-day transfer to a penal colony beyond the Arctic Circle.

The Kremlin critic's whereabouts had been unknown for more than two weeks, but he is now in a penal colony in Russia's far north and has been visited by his lawyer, his supporters said.

"Don't worry about me. I'm fine. I'm totally relieved that I've finally made it," Navalny wrote on X after arriving at the colony nicknamed "Polar Wolf".

"I'm still in a good mood, as befits a Santa Claus," he said, referring to his winter clothing of sheepskin coat and fur hat and the beard he grew out during his transportation.

The US State Department said it remained "deeply concerned for Mr. Navalny's wellbeing and the conditions of his unjust detention."

Navalny mobilised huge anti-government protests before being jailed in 2021 after surviving an assassination attempt by poisoning.

He has spent most of his detention at the IK-6 penal colony in the Vladimir region, some 250 kilometres (155 miles) east of Moscow.

A court in August extended his sentence to 19 years on extremism charges, and ruled he be moved to a harsher "special regime" prison for particularly dangerous prisoners.

Allies said his transfer could be linked to the upcoming presidential election in March, ahead of which many Kremlin critics have been jailed or fled.

"Right from the start it was clear that the authorities want to isolate Alexei, especially ahead of the elections," said Ivan Zhdanov, who manages Navalny's Anti-Corruption Foundation.

Navalny has been sent to "one of the furthest north and most remote colonies of all," Zhdanov added.

Ex-Gulag camp

According to the regional prison service website, the colony was built in the 1960s on the site of a camp that was part of the Stalin-era labour camp network, known as the Gulag. It can house up to 1,020 prisoners.

Inmates are put to work treating reindeer skins.

One major difference from his previous prison camp is that any letters will take much longer to reach Navalny.

Navalny posted on X that he arrived at the Arctic penal colony in the village of Kharp on Saturday and was visited by his lawyer on Monday.

Kharp is located above the Arctic Circle, over 1,900 kilometres (1,200 miles) northeast of Moscow. Its name means Northern Lights in the local Nenets language, and it is locked in the dark of the polar night in midwinter.

Navalny wrote that from his window "I can see the night, then the evening, and then the night again."

Prisoner transfers in Russia can take weeks as inmates are moved by train to far-flung facilities.

"I didn't expect anyone to find me here before mid-January," Navalny wrote, adding that he had seen little of his surroundings except for a snow-covered adjoining cell used as a yard and a fence outside his window.

"Unfortunately, there are no reindeer, but there are huge fluffy and very beautiful shepherd dogs," he said.

Temperatures in Kharp are expected to go down to minus 26 degrees Celsius (minus 14.8 Fahrenheit) in the coming days.

Vladimir putin