Five Ukrainian soldiers have been killed and seven others wounded in clashes with Russia-backed separatists in eastern Ukraine.

The Ukrainian Defense Ministry on August 23 said the casualties represented the biggest loss of life among soldiers in months.

Ukrainian officials said four of the soldiers were killed in a five-hour-long battle that broke out after separatists began firing mortar shells at government troops in the eastern Luhansk region.

However, the separatists said they were responding to attacks by government troops.

The AFP news agency quoted Defense Ministry spokesman Dmytro Gutsulyak as saying that a fifth soldier was killed in artillery fire elsewhere in the conflict zone.

Since April 2014, more than 10,300 people have been killed in fighting between Kyiv's forces and the separatists who control parts of the Luhansk and Donetsk regions.

Cease-fire deals announced as part of the Minsk accords -- September 2014 and February 2015 pacts aimed at resolving the conflict -- have failed to hold.

Ukrainian President Petro Poroshenko on August 23 said Ukrainian troops were attacked with high-caliber artillery.

"The Russians...are trying to attack the positions of our troops, violating the cease-fire," he said.

Poroshenko also apologized to the country for a promise he made in 2014 that he would be able to quickly end the conflict in the Luhansk and Donesk regions.

"People perceived it as an opportunity to end the war quickly," he said. "I am sorry to have created inflated expectations. I sincerely apologize for giving you hope that has not come true."

The United States and European Union have imposed a series of sanctions on Russia for its support for separatists in eastern Ukraine and for its seizure and illegal annexation of Ukraine’s Crimea region in March 2014.

Moscow has repeatedly denied that it has provided the rebels with weapons, troops, and ammunition despite overwhelming evidence that it has done so.

With reporting by AP, dpa, BBC, and AFP