Highlights: 

• 13 Ukrainian soldiers have been killed in a rebel attack at a military checkpoint in Donetsk Thursday.

• Insurgents attacked the checkpoint with rocket-propelled grenades and mortars.

• There has been no official word on rebel casualties, but local media said 25 insurgents were injured.

KIEV, May 22 -- At least 13 Ukrainian soldiers have been killed in a rebel attack at a Ukrainian military checkpoint in the eastern Donetsk region early Thursday, the country's acting President Alexandr Turchynov said.

"Our troops defended the town of Volnovakha and have not allowed insurgents to break through defenses, but unfortunately, our soldiers sacrificed their lives for Ukraine, tolerating mortar fire and heavy weapon fire," Turchynov said during his meeting with local religious figures.

Earlier in the day, the country's Defense Ministry had said heavy casualties were expected after insurgents attacked the checkpoint with rocket-propelled grenades and mortars, causing a Ukrainian armored vehicle to ignite and triggering a massive explosion.

There has been no official word on rebel casualties, but local media cited sources in the separatist camp as saying 25 insurgents were injured.

The incident comes just three days before the presidential election in Ukraine scheduled for May 25, triggering fears of possible disruption of the vote in the eastern part of the country.

According to the Central Election Committee, as of Thursday afternoon, 18 of 34 regional election commissions in Lugansk and Donetsk regions were blocked by separatists, who seek independence from the central government in Kiev.