A rebel leader said they would continue to fight and would not join further peace talks – but left unclear whether they would respect this week’s agreement to pull back heavy weapons from the front line.
Nato says it has indications that a major rebel offensive is coming, due to heavy weapons moving in from Russia as they have prior to previous rebel pushes.
Meanwhile, the UN human rights agency says the overall death toll in the conflict has risen above 5,000 as fighting escalates.
Separatist leader Alexander Zakharchenko said rebel fighters would continue to fight for more territory, and are advancing in five directions to push back Ukrainian government forces to the limits of the eastern Donetsk region.
“Attempts to talk about a ceasefire will no longer be undertaken by our side,” Zakharchenko said.
Any rebel advances would further undermine a tentative peace deal forged this week in Berlin at a meeting of foreign ministers from Russia, Ukraine, France and Germany.
Those negotiations concluded with an agreement to uphold a demarcation line defined in September after talks in the Belarusian capital, Minsk.
The plan calls for Ukrainian troops and Russian-backed separatists to pull back their heavy arms nine miles on either side of the line.
Ukrainian president Petro Poroshenko appeared to still be holding out hope for a new ceasefire, but said stern retribution would await anybody violating the peace.