Russian opposition leader Alexei Navalny attends a protest rally in Moscow. File photo

A Russian court on Tuesday found jailed Kremlin critic Alexei Navalny guilty of additional charges and extended his sentence to nine years in a higher security prison as Moscow seeks to wipe out remaining pockets of dissent.

The sentencing came on the 27th day of what Moscow calls its "special military operation" in pro-Western Ukraine, with thousands killed and some 10 million displaced.

Germany has condemned the latest verdict against Russian opposition leader Alexei Navalny.

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In a statement, Germany’s Foreign Ministry said Tuesday that the new prison sentence "is part of the systematic instrumentalization of the Russian judicial system against dissidents and the political opposition.”

The ministry noted that the latest trial took place largely behind closed doors and accused Russian authorities of breaching fundamental principles of rule of law while failing to present any credible evidence against Navalny.

"The German government renews its demand for Navalny’s immediate release,” it said.

Following the verdict, President Vladimir Putin's most vocal domestic critic who has been behind bars for more than a year said the Russian leader was afraid of the truth.

"Putin is afraid of the truth, I have always said this. Fighting censorship, relaying the truth to the people of Russia always remained our priority," the 45-year-old opposition politician said in a post on Instagram after the sentencing.

With his trademark sense of humour, Navalny took the extension of his jail time on new charges of embezzlement and contempt of court in stride, joking that his "space flight is being extended — my ship has gotten caught in a time loop."

He also urged Russians to resist "these war criminals".

He stood trial inside his penal colony in the town of Pokrov outside Moscow.

"Navalny committed fraud — the theft of property by an organised group," judge Margarita Kotova said, according to an AFP reporter present at the hearing.

The judge also found Navalny guilty of the less severe charge of contempt of court.

Navalny will serve his new sentence in a strict-regime penal colony, which will place him in much harsher conditions.

His lawyers, Olga Mikhailova and Vadim Kobzev, were detained by police following the verdict but were later freed.

Navalny's defence team said the new sentence replaces the one he is currently serving, meaning that he will not be freed for another eight years, instead of a year and a half.