Russian President Vladimir Putin has accused Ukraine's government of ordering a major new offensive against pro-Russian rebels in the separatist east, in spite of the latest ceasefire agreement.
Putin said on Friday that "artillery is being used, rocket launchers and aviation, and it is used indiscriminantly and over densely populated areas". Kiev is responsible for the deaths, he said.
Meanwhile, NATO said it had indications that a major rebel offensive was being planned due to heavy weapons moving in from Russia, as they have prior to previous rebel pushes.
On Friday, a rebel leader in the 'Donetsk People's Republic' (DPR) said that the separatists would reject truce talks and go on to take over the entire Donetsk region - half of which is currently controlled by Ukrainian authorities.
"So we made the decision not to wait until the Ukrainian army starts an offensive and not allow them to make battle formations ... [or] allow them to harm us in any way," said Alexander Zakharchenko, a rebel leader of the DPR group.
"We will attack until we reach the borders of Donetsk region."
Zakharchenko said another rebel group, the "Luhansk People's Republic", was also on the offensive, pushing the army in two directions.
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.
"If the enemy doesn't want to abide by the ceasefire ..., we will smash them in the teeth."
Those negotiations concluded with an agreement to uphold a demarcation line defined in September after talks in the Belarussian capital, Minsk.
The plan calls for Ukrainian troops and Russian-backed separatists to pull back their heavy arms 15km 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.
Petro Poroshenko, Ukraine's president, said on Thursday he was still holding out hope for a new truce, but said the army would respond to any violations.
"If the enemy doesn't want to abide by the ceasefire, if he doesn't want to put an end to the suffering of peaceful people, Ukrainian villages and towns, we will smash them in the teeth," he said.
The UN human rights agency has said the overall death toll in the conflict has risen above 5,000.