Jailed Russian opposition leader Alexei Navalny has been moved to a penal colony in the Arctic, allies said on Monday after over two weeks during which his whereabouts were unknown.

Authorities transferred Russia's most prominent opposition politician to an isolated penal colony three months before a presidential vote expected to easily hand Vladimir Putin a fifth term.

"We have found Alexei Navalny," his ally, Kira Yarmysh, said on social media.

"He is now in IK-3 in the settlement of Kharp in the Yamalo-Nenets Autonomous District," Yarmysh added. "His lawyer visited him today. Alexei is doing well."

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.

Washington welcomed reports that the dissident was finally located alive, but slammed Moscow for its "malicious targeting of Navalny and the more than 600 other political prisoners Russia has imprisoned."

In a statement on Christmas Day, the State Department said: "We remain deeply concerned for Mr. Navalny's wellbeing and the conditions of his unjust detention."

It said it joined Navalny's family and supporters "in calling for his immediate release, without conditions," and for Russia to "end its escalating repression of independent voices in Russia, who are subject to relentless harassment and intimidation for exercising their human rights."

Navalny mobilised huge anti-government protests before being jailed in 2021, after surviving an attempt to assassinate him by poisoning.

He first spent most of his detention at the IK-6 penal colony, some 250 kilometres east of Moscow in the Vladimir region.

A court extended his sentence to 19 years on extremism charges.

It also ruled he be moved to a harsher special regime prison, usually housing particularly dangerous prisoners.

After weeks of uncertainty, Navalny was found at IK-3 -- "Polar Wolf " -- a strict regime colony.

He could be moved to the nearby penal colony No.18 -- "Polar Owl" -- which has an even harsher regime.

It is common for transfers to take weeks in Russia, as prisoners are slowly moved by rail between far-flung facilities.

But the complete lack of news on his whereabouts had spurred concerns from allies, rights groups and Western governments.

Navalny's allies said they had sent requests to over 600 detention centres.

"There was a complete ban of information about him," Zhdanov said.

Allies believe the Kremlin aimed to further isolate the vocal critic.

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

Russia will hold a presidential vote in March 2024, with Putin the undisputed favorite.

Several Kremlin-friendly parties are due to put forward candidates for the vote in March, but the real opposition has been sidelined.

Moscow has for years weakened Russia's independent politicians and activists, a clampdown that accelerated after the Russia-Ukraine war broke out in 2022.

Navalny's movement in particular has been targeted by Kremlin repression.

Even before the Ukraine conflict, Navalny's organisation was declared extremist, which facilitated the prosecution of members and supporters.

Several regional heads of the Anti-Corruption Foundation were jailed.

Among them was Lilia Chanysheva, Navalny's ally in the central Bashkortostan Republic, who was handed seven and a half years in prison this summer.

Many high-profile allies have fled, including Maria Pevchikh, who runs the Anti-Corruption Foundation from abroad and was targeted by a Russian arrest warrant last week. (AFP)