"The car was traveling with a large white flag, but that did not stop the fighters," spokesman Andriy Lysenko said in a televised briefing.
The deaths follow those of around 17 refugees, who were killed when rocket fire hit a convoy of buses leaving Luhansk on Monday. Both sides blamed each other for the attack. Five Ukrainian servicemen were also killed overnight, Lysenko said.
Living conditions in many parts of eastern Ukraine have worsened dramatically — Luhansk has been largely cut off for weeks and is in its 19th day without water and regular supplies of electricity, which have hit mobile and landline phone connections. The UN has put the death toll in the conflict at over 2,000, including civilians and combatants. That figure has nearly doubled since late July, when Ukrainian forces stepped up their offensive and the conflict spread to major urban areas.
Lysenko also said a SU-24M military plane had been shot down by rebels on Wednesday, correcting an earlier statement which said a SU-25 ground attack plane had been hit. Both pilots ejected safely, he said.