Ukrainian army units which had been trapped by separatists on the border with Russia broke out of the blockade on Friday and rejoined government forces, but 15 soldiers and border guards were killed in the operation, the Ukrainian military said.
Military sources quoted by Ukrainian media said Ukrainian units had been effectively encircled by the rebels on a section of the border with Russia south of the town of Luhansk and east of the main regional city of Donetsk.
After government forces opened up an escape corridor, the trapped units were able to force their way out, military sources quoted by the media said.
"Seven service staff and eight border guards were killed and 79 injured," military spokesman Andriy Lysenko told journalists.
Government forces say they are gradually tightening the noose around the heavily armed pro-Russian separatists whom they have been battling since mid-April in a conflict which the United Nations High Commissioner for Human Rights says has cost the lives of more than 1,100 people in all, including government forces, rebels and civilians.
The latest military deaths from the fighting in the Russian-speaking east of Ukraine push the death toll among government forces alone to more than 400.
The Kiev government and its Western allies accuse Russia of seeking to destabilise Ukraine and arming the rebels, who have declared independent "people's republics" in the two main industrial regions. Moscow denies involvement.
In the biggest Russia-West confrontation since the end of the Cold War, the United States and European Union have imposed sanctions on Russia. Moscow has retaliated with a sweeping ban on imports of many Western foodstuffs.
Fighting has intensified since the shooting down of flight MH17 on July 17, killing all 298 passengers and crew, an act which the West laid at the door of the rebels. Russia and the rebels blame the disaster on Kiev's military offensive.
An international crew of experts retrieving debris and victims' belongings from the crash site which is spread over a wide area in the conflict zone halted their work on Wednesday because of dangers to their safety from fighting.
The Kiev government on Friday announced it would continue to observe a ceasefire in the area, contradicting an earlier announcement on Thursday.