Ukrainian authorities and Russia-backed separatists in the war-torn east of the country began a prisoner exchange on Sunday in a frontline operation that has stirred controversy. The exchange came after Russian President Vladimir Putin and Ukrainian leader Volodymyr Zelensky held their first face-to-face talks in Paris on December 9 and agreed measures to de-escalate Europe’s only active war. As part of the swap, Kiev was expected to hand over to separatists several riot policemen suspected of killing protesters during a pro-Western uprising in 2014, in a move that sparked public anger.