Russia said Sunday that trains carrying more bodies of Ukrainian soldiers were due to depart towards the Ukrainian border, escalating a spat with Kyiv over a prisoner of war swap.

MOSCOW/KYIV — A large-scale prisoner exchange between Russia and Ukraine — the only concrete outcome of peace talks — was thrown into doubt on Saturday after Moscow and Kyiv accused each other of delaying and thwarting the swap.

Russia and Ukraine began a major prisoner exchange, swapping hundreds of soldiers and civilians in the first phase of an exchange that was a moment of cooperation in otherwise failed efforts to reach a ceasefire in the 3-year-old war.