WASHINGTON, United States - The United States expressed serious concern Monday about the detention of top Russian opposition figure Alexei Navalny, who has been transferred to an Arctic prison, as it urged Moscow to end "escalating repression" of dissidents.

Alexei Navalny

The State Department said it welcomed reports that Navalny -- jailed since 2021 on various charges including "extremism" -- has been located after his whereabouts were unknown for more than two weeks.

"However, we remain deeply concerned for Mr. Navalny's wellbeing and the conditions of his unjust detention," a spokesperson said in a statement, adding US officials condemn the "malicious targeting" of the 47-year-old.

Washington joins Navalny's family and supporters "in calling for his immediate release, without conditions," and "we call on the Russian government to end its escalating repression of independent voices in Russia," the spokesperson said.

Navalny is Russian President Vladimir Putin's main political opponent. The dissident was barred from running in elections in 2018 due to an old fraud charge that his allies said was politically motivated.

He received a 19-year prison sentence in 2021 on charges of "extremism" after surviving a poisoning attempt on his life, for which he blamed Putin.

Navalny, who has experienced major weight loss in prison, was transferred from a facility east of Moscow to a remote penal colony above the Arctic circle with harsher conditions and little contact with the outside world, according to Ivan Zhdanov, who manages Navalny's Anti-Corruption Foundation.

"We have conveyed to the Russian government that they are responsible for what happens to Mr. Navalny in their custody, and the international community is watching closely," the State Department said in its statement, as it also called out Moscow for holding "more than 600 other political prisoners."