A Russian court today sentenced a journalist to six years in prison for comments critical of Moscow's offensive in Ukraine, officials said, as part of an ongoing crackdown on dissent.

The verdict is the latest in a series of high-profile rulings under new legislation that critics say is designed to criminalise criticism of the military intervention.

Maria Ponomarenko, 44, was sentenced for spreading information she knew to be false about Moscow's army, said the Investigative Committee, which probes major crimes.

The journalist was sentenced in the southern Siberian city of Barnaul, where she worked for the RusNews website.

She was prosecuted for a post on social media last March related to an attack on a theatre in Ukraine's port city of Mariupol that came under Russian control after a long siege.

Kyiv and its Western allies blamed Moscow for the death of hundreds of civilians in the attack, which Russia denies.

Prosecutors had requested a jail term of nine years.

Ponomarenko was arrested last April in Russia's second city of Saint Petersburg before being transferred to Barnaul.

Her lawyer has raised alarm over Ponomarenko's deteriorating mental health and requested that she receives treatment, the OVD-Info law-enforcement monitoring group reported.

After the Kremlin ordered troops into Ukraine nearly a year ago, Russia introduced new legislation criminalising what authorities consider to be false or damaging information about the Russian army and the offensive.