By Staff, Agencies

More than 1,000 Ukrainian servicemen who surrendered in the besieged Azovstal steel plant in the southern port of Mariupol have been transferred to Russia for investigation, in a move that could undermine already troubled peace talks between the two sides.

"More than 1000 people from Azovstal were brought to Russia. Law enforcement bodies are working with them closely," Tass news agency reported on Tuesday, citing a Russian law enforcement source, without providing further information.

The source also said that later on, more Ukrainian prisoners would be sent to Russia.

Back in May, Moscow said it had managed to fully liberate Azovstal, a giant fortress-like steel plant in Mariupol.

It said more than 2,400 people who had been stuck in the complex for almost a month, including Ukrainian servicemen and “members of the neo-Nazi Azov unit”, laid down their arms and surrendered.

At the time, Moscow released no information about where the surrendered soldiers were being evacuated to, but Ukrainian authorities hoped they could be released as part of a prisoner swap.

Russian lawmakers pushed plans to declare Azov regiment fighters "Nazi criminals" who must not be included in the prisoner swap with Ukraine.

The office of Russia's prosecutor general asked the country's apex court to declare the unit a "terrorist organization."

The Azov regiment, set up as a volunteer militia in 2014, has had close ties to the far right.

Meanwhile, Ukraine says Russia has so far handed over the bodies of 210 Ukrainian soldiers killed in the battle for Mariupol, most of whom were among the last holdouts in Azovstal.

"The process of returning the bodies of the fallen defenders of Mariupol is under way. To date, 210 of our troops have been returned - most of them are heroic defenders of Azovstal," Ukraine's defense intelligence directorate said in a post on Twitter on Tuesday.

"Work continues on bringing home all of the captured Ukrainian defenders," the directorate added.

The families of Ukraine's Azov unit of the National Guard had earlier reported the return of some bodies.

Last week, Ukraine's Ministry of Reintegration of the Temporarily Occupied Territories announced an exchange of 160 bodies between Russia and Ukraine.