Russia and Ukraine discussed further prisoner swaps on Wednesday at a brief session of peace talks in Istanbul, but the sides remained far apart on ceasefire terms and a possible meeting of their leaders.

Ukraine and Russia launched air attacks along each other's Black Sea coasts early on Thursday, hours after brief direct talks between them failed to make any progress on steps to end nearly three-and-a-half years of war.

The warring sides remain far apart after the third round of U.S.-brokered peace negotiations on Wednesday. But there was progress on humanitarian issues.