A spokesman for the army in Kiev said four servicemen were killed and 12 injured in mortar fire exchanges across the eastern separatist province of Donetsk.
"The situation is getting worse," Oleksandr Motuzyanyk told a daily press briefing. "The rebels' attacks have intensified."
Pro-Russian militia commander Eduard Basurin told AFP that Ukrainian shells had killed one civilian and wounded another 12 in the rebels' de facto capital of Donetsk and their nearby stronghold of Gorlivka.
Moscow denies sparking the uprising in reprisal for the February 2014 unseating of a Russian-backed president by a pro-Western leadership that is seeking future membership in the European Union and possibly NATO.
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But Ukrainian President Petro Poroshenko said in an address marking 24 years of post-Soviet independence on Monday that 9,000 Russian servicemen were among the 40,000 fighters of the separatist force.
The US State Department on Monday also insisted that it was "the combined Russian separatist forces, and not Ukrainian forces, who are initiating aggressive activities" that resumed with renewed vigour earlier this month.
Poroshenko met German Chancellor Angela Merkel and French President Francois Hollande for crisis talks that notably did not include Russia's Vladimir Putin.
The two EU heads of state still see the Minsk deal as the only solution to a conflict that has shattered Moscow's relations with most Western powers and renewed fears of Russia's invasion of former communist eastern European states.
"Everything must be done so the ceasefire can become a reality," Merkel said heading into Monday's meeting with Poroshenko and Hollande.