Russia-backed separatists killed at least five people in the region of Donetsk, officials said Tuesday.
Officials at the rebel-run Donetsk city hall also said at least 13 people were wounded in the fierce outbreak of shelling over the previous 24 hours.
Fighting has raged in the self-declared separatist regions of Donetsk and Luhansk, especially around the Ukrainian-controlled town of Debaltseve, since peace talks in Belarus between Ukraine's government and the separatists collapsed Saturday.
There are fears the violence could soon escalate. On Monday, a pro-Russian separatist leader announced a "general mobilization" with the aim of bolstering separatist armed forces to as many as 100,000 fighters.
Meanwhile, the United States says it has not made a decision on whether to provide Ukraine with lethal military assistance in its fight against the separatists.