Jailed Russian opposition leader Alexei Navalny, whose whereabouts were not known for over two weeks, has been moved to a penal colony in the Arctic, allies said Monday.

The disappearance of Russia's most prominent opposition politician, who mobilised huge protests before being jailed in 2021, had spurred concerns from allies, rights groups and Western governments.

It signalled a likely prison transfer, which can take weeks in Russia as prisoners are slowly moved by rail between far-flung facilities. 

"We have found Alexei Navalny. He is now in IK-3 in the settlement of Kharp in the Yamalo-Nenets Autonomous District," his ally, Kira Yarmysh, said on social media.

"His lawyer visited him today. Alexei is doing well," Yarmysh added.

The district of Kharp, home to about 5,000 people, is located above the Arctic Circle.

It is "one of the most northern and remote colonies," said Ivan Zhdanov, who manages Navalny's Anti-Corruption Foundation.

"Conditions there are harsh, with a special regime in the permafrost zone" and very little contact to the outside world, Zhdanov said.

Navalny was first jailed after surviving an attempt to assassinate him by poisoning.

A court extended his sentence to 19 years on extremism charges, and ruled that he be moved to a more secure, harsher prison.

"From the very beginning, it was clear that authorities wanted to isolate Alexei, especially before the elections," Zhdanov also said.

Russia will hold a presidential vote in March 2024, expected to easily hand President Vladimir Putin a fifth term.

Moscow has for years sidelined opposition figures from elections and political life, a clampdown that accelerated after the Kremlin ordered Russian troops into Ukraine in 2022.