VIENNA (AP) — Poland has extradited to Austria a man suspected of war crimes after he allegedly killed civilians and enemy troops after they had surrendered while fighting for Ukrainian forces, authorities said Tuesday.

The 25-year-old Austrian man allegedly carried out the killings last year against mostly ethnic Russian separatists who are being supported by Russia in the conflict in eastern Ukraine. Polish authorities said he also allegedly killed injured troops.

More than 10,000 people have died since the hostilities flared in eastern Ukraine in April 2014.

The suspect denies the accusations. He is being identified only as Benjamin F. in keeping with Austrian and Polish confidentiality rules.

He was handed over by Polish officials at Warsaw's Chopin airport on Monday. State prosecution spokesman Erich Habitzl said the man is being held in a police cell while authorities decide whether he should be detained longer.

The spokesman for the court in Lublin, in eastern Poland, Dariusz Abramowicz, said the man had been wanted on a European arrest warrant and was detained in late April at the Dorohusk border crossing as he was trying to leave Poland for Ukraine.

Abramowicz said Poland has stepped up the controls on its eastern border, which is also the European Union's outside border.

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Monika Scislowska in Warsaw contributed to this report.