Washington DC - The United States has called for the immediate release of jailed Kremlin critic Alexey Navalny, as the de facto leader of Russia's political opposition was returned to solitary confinement.

The US government was joining Navalny's family and his colleagues and supporters around the world in demanding his release, State Department spokesperson Ned Price said in Washington on Friday.

"The United States is deeply concerned by the Russian Government's escalating, arbitrary interference with Aleksey Navalny's rights," a statement read.

On Wednesday, Navalny tweeted that he had been ordered to spend 15 days in solitary confinement, which he said was the fourth time he had been placed in isolation at his prison camp in the town of Melekhovo, about 160 miles north-east of Moscow.

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Price accused the Russian authorities of interfering with Navalny's communication with his legal counsel. "This interference, along with his repeated diversion to solitary confinement for minor alleged infractions, is further evidence of politically motivated harassment."

In mid-August, Navalny issued a call from his prison cell for the West to impose tougher sanctions on Russian oligarchs. Russia's best-known opposition figure has long been calling for tough sanctions against those who support the Kremlin.

Considered by most to be a political prisoner, Navalny is currently serving a nine-year prison sentence for alleged fraud, a conviction widely seen as politically motivated.

Navalny miraculously survived a poisoning attempt on a domestic flight in Siberia in August 2020, after which he was airlifted to Germany for months of medical treatment before voluntarily returning to Russia, where he was immediately arrested for breaking the terms of his parole.