Six Ukrainian soldiers were killed in a rebel ambush between the insurgent bastions of Slavyansk and Kramatorsk in the east of the country yesterday.
The defence ministry said a military convoy was attacked by a group of about 30 men who fired heavy weapons at the troops, triggering prolonged clashes that left another eight wounded.
The clashes came as Germany's foreign minister tried to broker talks between Ukraine's central government and pro-Russia separatists.
Speaking at Kiev's main airport, Berlin envoy Frank-Walter Steinmeier said Germany supported Ukraine's efforts to arrange for a dialogue between the central government and its opponents in the eastern Donetsk and Luhansk regions that form the nation's industrial heartland.
Pro-Russia insurgents have seized government buildings and clashed with government forces in eastern Ukraine in the past month and are holding some journalists and others hostage.
Steinmeier voiced hope for a quick release of the hostages, the handover of occupied buildings and stressed the importance of holding Ukraine's presidential vote as planned on May 25.
The Ukrainian government and the West have accused Russia of fomenting the mutiny in the east to derail Ukraine's presidential vote and possibly grab more land. Steinmeier's trip is part of the road map for settling Ukraine's crisis laid out by the Organisation for Security and Cooperation in Europe, a trans-Atlantic security group.
Russia called yesterday for a swift implementation of the OSCE plan, saying its demand to end violence means that the central government in Kiev should stop its military operation to recapture buildings in the east, lift its blockade of cities and towns, pull its forces from eastern regions and release all political prisoners.
"We are demanding that they stop intimidating civilians by using force or threatening to use it," Russia's Foreign Ministry said.
It added that it expects separatists in Ukraine's Donetsk and Luhansk regions to respond in kind if Kiev does all that.
"(The road map) creates conditions for launching a broad national dialogue involving all political forces and regions of Ukraine, aimed at reconciliation and a comprehensive constitutional reform intended to stop the nation from sliding further to catastrophe," the ministry said.
Russia also urged the United States and the European Union to persuade authorities in Kiev to prioritise discussions about giving more powers to Ukraine's regions ahead of the country's presidential vote.
The separatists held a referendum on Sunday and claimed that about 90 per cent of those who voted in Donetsk and Luhansk backed sovereignty.
The two regions declared independence on Monday and those in Donetsk even asked to join Russia.
Ukraine's acting president called the vote a sham and Western governments said it violated international law.
Additional reporting by Agence France-Presse