A rebel leader said they will continue to fight and won't 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 U.N. 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 will 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 cease-fire 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 15 kilometers (9 miles) on either side of the line, though there was no agreement on a withdrawal of troops.
The urgency for a fresh settlement peaked over the weekend as battles intensified over control over the airport on the northern edge of the separatist stronghold of Donetsk. Clashes culminated with rebels taking control over the terminal, although fighting is continuing in nearby settlements.
Separatist officials say Ukrainian forces are still targeting the airport.
Ukrainian President Petro Poroshenko appeared Thursday to still be holding out hope for a new cease-fire, but said stern retribution would await anybody violating the peace.
"If the enemy doesn't want to abide by the cease-fire, if he doesn't want to put an end to the suffering of peaceful people, Ukrainian villages and town, we will smash them in the teeth," Poroshenko told a meeting of top defense officials.
Mstyslav Chernov, Associated Press