Seven Ukrainian soldiers were killed when their armoured column was ambushed by pro-Russian separatists in eastern Ukraine today, the defence ministry and state security service said.
Seven more soldiers were wounded in the ambush in the town of Kramatorsk.
It was the biggest single loss of life by the Ukrainian army since soldiers were sent into the mainly Russian-speaking east of the country to break up armed separatist groups.
Separatists have seized control of towns and public buildings to push their demands for autonomy.
The ministry said an armoured column came under fire as it approached a bridge near a village 20km from Kramatorsk.
Kramatorsk is one of several hot spots in the region where the army has had only limited success against the separatists.
About 30 rebels, who had taken cover among bushes along a river, attacked with grenade-launchers and automatic weapons, the ministry said.
Meanwhile, German Chancellor Angela Merkel has called for planned roundtable talks in crisis-hit Ukraine to be as "representative" as possible.
However, she warned there was no place for those who use violence.
Ms Merkel told reporters that she and the German government were working to ensure Ukraine's presidential elections can take place on 25 May and she urged all parties, including Russia, to help.
"We believe that the possibility of roundtables under the guidance of the OSCE (Organisation for Security and Cooperation in Europe) offers here a good possibility," she said of the Ukrainian national talks set for tomorrow.
"The more representative the roundtables are, the better that is," she added.
"But it's clear of course too that only those can be welcome who are ready and show credibly that they don't achieve their goals with violence," Ms Merkel said.
Ms Merkel also termed Sunday's independence referendum in eastern Ukraine, claimed by rebels to have been a resounding victory, as "illegal" and said she therefore was not so interested in the result.
"I'm waiting for the elections on May 25," she said.
German Foreign Minister Frank-Walter Steinmeier was in Ukraine today to push the Kiev authorities and pro-Moscow rebels to come together at the negotiating table after the OSCE drew up a roadmap aimed at easing tensions.
Kiev is hosting a roundtable meeting tomorrow involving the government, parliament and regional leaders but notably not any separatist representatives.