A military convoy was attacked near a river in a village between the rebel bastions of Slavyansk and Kramatorsk by a group of about 30 men who fired heavy weapons including grenade launchers, the ministry said.
The troops responded, triggering "prolonged clashes" that left six soldiers dead and eight wounded, the ministry said.
A spokeswoman for the Ukrainian security services said later the death toll had risen to seven.
It is not known how many police or other members of the security forces have been killed.