The announcement appears to preclude any kind of top-level meeting or breakthrough ahead of the 50-day deadline for an agreement given by President Donald Trump.

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.

Officials from Russia and Ukraine ended a third round of formal negotiations in Istanbul with an agreement to swap more prisoners but little sign of progress on a deal to halt the war.