Polish authorities say they have arrested an Austrian man suspected of committing war crimes in eastern Ukraine, though there were conflicting reports about whether he fought with Kyiv's forces or alongside Russia-backed separatists.

The Polish border service said in an April 30 statement that the 25-year-old man was arrested at Austria's request two days earlier as he was trying to cross into Ukraine at the Polish border post of Dorohusk.

The man is suspected of participating in the fighting in eastern Ukraine, where he allegedly "killed the wounded and prisoners," the statement said.

Both the killing and the torture of prisoners of war constitute war crimes under the Geneva Conventions.

The war between Kyiv's forces and Russia-backed separatists has killed more than 9,900 people since April 2014.

There were contradictory reports about which side the Austrian man fought for. Polish border police spokesman Dariusz Sienicki was quoted by the AFP news agency as saying that "he was a member of a separatist group."

But Erich Habitzl, a spokesman for Austrian prosecutors, said on April 30 that the man had fought on the Ukrainian side, including in the brutal, protracted battle for Donetsk airport.

Rights groups and international organizations have accused both sides of arbitrary killings and torture.

A UN report last year found evidence of arbitrary killings by Russia-backed separatists, Ukrainian troops, police forces, and volunteer battalions of fighters.

The report cited executions of prisoners by both sides and the indiscriminate shelling of civilian areas using imprecise weapon systems.

Austria is seeking the suspect's extradition, Polish border police said.

With reporting by AFP, dpa, and Reuters