KYIV (Reuters) -Russia's claims that Ukraine is delaying exchange of soldiers' bodies are untrue, Ukrainian officials said on Saturday, urging Moscow to stop "playing dirty games" and return to constructive work. Kremlin aide Vladimir Medinsky said on Saturday that Ukraine had unexpectedly postponed exchanging prisoners of war and accepting the bodies of killed soldiers for an indefinite period. Russia and Ukraine held the second round of peace talks in Istanbul on Monday where they agreed to exchange more prisoners - focusing on the youngest and most severely wounded - and to return the bodies of 12,000 dead soldiers.