Kremlin-installed courts in Russian-controlled regions of Ukraine have sentenced three Ukrainian servicemen, including a human rights activist, to lengthy jail terms for allegedly mistreating civilians, Russian investigators said Friday.

"The supreme courts of the Donetsk and Lugansk People's Republics issued sentences in three criminal cases against Ukrainian citizens Viktor Pokhozey, Maksym Butkevych and Vladislav Shel," Russia's Investigative Committee said in a statement.

Moscow last year declared the Donetsk and Lugansk regions in eastern Ukraine as part of Russia following referendums denounced by Kyiv and the West.

"All of them were found guilty of mistreating the civilian population and using prohibited methods (of fighting) in an armed conflict," the statement added.

Butkevych, a Ukrainian rights activist and co-founder of the independent Hromadske radio, and Shel were also convicted of attempted murder.

Butkevych joined the Ukrainian army in March, shortly after Russia launched its offensive in Ukraine.

Investigators claimed Butkevych injured two civilians when he "fired an anti-tank grenade launcher" at the entrance of a residential building in Severodonetsk, a city in Lugansk region now under Russian control.

He was sentenced to 13 years in jail.

Shel was handed 18 years and six months in jail. Investigators said he fired shots at a man to "intimidate civilians" in Mariupol, a port city in southern Ukraine that fell into Russian hands after a weeks-long siege.

Pokhozey, a commander of the Azov battalion which defended the Azovstal steelworks in Mariupol, was jailed for 8 years and six months for allegedly beating a woman in Mariupol.

They will serve their sentences in a strict regime penal colony, investigators said.