A Russian court on Monday sentenced leading science fiction writer Dmitry Glukhovsky to eight years in prison in absentia for denouncing Moscow's Ukraine offensive on social media.

Glukhovsky, who lives outside Russia, is best known for his novel Metro-2033, that has also been used as a basis for a popular video game.

The 44-year-old, who Moscow has already labelled a "foreign agent", was found guilty of spreading "fake information" about the Russian army, the TASS news agency reported.

He was sentenced under a law adopted shortly after President Vladimir Putin sent troops to Ukraine that has been used to stifle criticism of the offensive.

Glukhovsky has consistently denounced Moscow's military campaign against its neighbour.

After a Russian strike on Odesa late last month, he posted a black square with the word "Odesa" coupled with a heart sign on his Instagram page with the caption: "Bitches."

On the anniversary of the start of the full-scale conflict in February, the novelist said on social media: "As a year ago, I am completely convinced that this war is just as ruinous for Russia and our people as it is deadly and destructive for the nation that I love that is Ukraine."

Russia has led an unprecedented crackdown on dissent as its troops fight in Ukraine.

Glukhovsky has also posted in support of jailed opposition politician Alexei Navalny, who last week was handed a 19-year prison sentence.

On Monday, Navalny's team said the Putin critic was again placed in solitary confinement, this time for two weeks.

"It is the 18th time," his team wrote on Telegram.

Navalny, who has denounced the Ukraine offensive from behind bars, was sentenced to serve his long sentence in a "special regime" penal colony on Friday.