Seven Ukrainian soldiers have been killed fighting Russian-backed rebels in the east of the country over the past 24 hours, the military says.
Ukraine said the troops were killed by shelling, mostly in the Luhansk region, that also left 24 soldiers injured.
Violence in eastern Ukraine is at its worst level for months with a series of deadly attacks over the past week.
On Saturday, a series of rocket attacks left 30 dead and many more injured in the Black Sea city of Mariupol.
The heaviest fighting overnight into Monday was around the town of Debaltseve, Ukrainian military spokesman Vladyslav Seleznyov said.
Rebel leader Alexander Zakharchenko said on Saturday that separatist forces intended to surround the government-held town.
He also initially said his troops had begun an offensive to retake the strategically important city of Mariupol, but later backtracked and blamed Ukrainian forces for the deaths there.
OSCE observers said analysis of a crater in Mariupol showed that the rockets were fired from the direction of rebel-held territory. Ukrainian President Petro Poroshenko said intercepted radio and telephone conversations proved separatists were behind the attacks.
More than 5,000 people have been killed in fighting since the rebels seized a large swathe of Donetsk and Luhansk regions last April, according to UN estimates. More than a million people have been displaced.
A ceasefire agreed as part of a peace deal signed in Minsk in September has been repeatedly violated by both sides.
Ukraine and the West have accused Russia of arming the rebels and sending its troops into Ukrainian territory.
Russia has denied directly arming the separatists, and blames Ukraine for the surge in fighting.
On Monday, Russian Foreign Minister Sergei Lavrov said Kiev had tried “to derail the peace process… again and again” and warned that attempts to use force “to suppress the south-east [of Ukraine]” would “lead nowhere”.
The OSCE said its permanent council would meet in Vienna on Monday “in light of the rapid deterioration of the situation in eastern Ukraine”.
Ukraine The Human Cost
- Some 5.2 million people live in conflict-affected areas and 1.4 million are considered “highly vulnerable and in need of assistance”
- More than a million people have fled their homes with 633,523 living as displaced persons within Ukraine and 593,622 living outside Ukraine, mostly in Russia
- More than 5,000 people have been killed in the fighting and more than 10,300 injured