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.

A Russian defense official Sunday said trains carrying the remains of Ukrainian soldiers would be dispatched to an area near the Ukrainian border, stepping up tensions over a prisoner swap between Moscow and Kyiv.

Ukrainian President Volodymyr Zelenskiy vowed on Sunday to press on with prisoner exchanges with Russia and said any failure by Moscow to uphold humanitarian accords cast doubt over U.S. and other efforts to end the more than three-year-old conflict.