Russia on Thursday handed a long jail term to a Siberian man accused of state treason and terrorism over plans to join Russian units fighting for Ukraine.

This is the latest arrest of Russian citizens accused of wanting to fight as part of pro-Ukrainian armed groups, which have led several deadly incursions into Russia.

The suspect "planned to take part in combat on the side of the Ukrainian armed forces," Russian state-run agency TASS quoted the FSB as saying.

Law enforcement detained the 26-year-old in the central Kemerovo region as he was making his way to Ukraine, the report said.

"He was motivated by the fact that, since the start of the operation in Ukraine, he did not support the politics of Russia's leadership," the FSB statement said.

TASS reported that he posted symbols of the Freedom of Russia Legion online -- one of the anti-Kremlin Russian unit fighting for Kyiv.

Moscow has banned it as a terrorist organisation.

Russia regularly prosecutes people for collaborating with Ukraine.

In the southern Ukrainian Zaporizhzhia region occupied by Russia, Moscow-installed authorities on Thursday sentenced a man to 11 years in prison for spying for Ukraine.

TASS said he "gathered and passed information on movements and directions of Russian troops" and "collected information on industrial and energy infrastructure" in the Zaporizhzhia region.