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.