KYIV, June 19 (Reuters) - Two top commanders of fighters who defended the Azovstal steel plant in Ukraine's southeastern port of Mariupol have been transferred to Russia for investigation, Russia's state news agency TASS reported.

Uncertainty has surrounded the fate of hundreds of fighters captured by Russian forces in May after a months-long siege of Mariupol. Moscow said at the time they were moved to breakaway Russian-backed entities in eastern Ukraine.

Citing an unnamed Russian law enforcement source, TASS said late on Saturday that Svyatoslav Palamar, a deputy commander of the Azov battalion, and Serhiy Volynsky, the commander of the 36th Marine Brigade of the Ukrainian Armed Forces, were moved to Russia.

Special forces officers transferred them from Donetsk, an eastern Ukrainian province that Moscow recognises as a pro-Russian republic, "to conduct investigative activities with them," TASS cited the source as saying.

"Other officers of various Ukrainian units were also transported to Russia."

Reuters could not immediately verify the report and there was no immediate reaction from Kyiv. Earlier this month, Ukraine said its intelligence services were in communication with the captured Azovstal steelworks fighters.

The Azov Battalion, an all-volunteer infantry military unit which formed in 2014 as an extreme right-wing volunteer militia to fight Russian-backed separatists, and the 36th Marine Brigade unit, were key in defending the steelworks.

Kyiv is seeking the handover of all the fighters in a prisoner swap with Moscow, but some Russian lawmakers want some of the fighters put on trial.

Russian agencies reported in early June that more than 1,000 Azovstal fighters were transferred to Russia to undisclosed locations for investigation.

The self-styled Donetsk and Luhansk people's republics are in the industrial Donbas region in eastern Ukraine, which Russia is fighting to remove from Kyiv's control.