(Adds rebel reaction, description of impact of rocket fire)
KIEV, Aug 18 (Reuters) - Rocket fire hit a convoy of buses carrying refugees from the east Ukrainian city of Luhansk on Monday killing an unknown number of people, Ukraine's military and rebels said, blaming each other for the strike.
Around 500 people a day have been fleeing Luhansk, a pro-Russian separatist stronghold that has been battered by months of fighting between rebels and Ukrainian forces and left almost entirely without water and electricity for more than two weeks.
"A powerful artillery strike hit a refugee convoy near the area of Khryashchuvatye and Novosvitlivka. The force of the blow on the convoy was so strong that people were burned alive in the vehicles - they weren't able to get themselves out," military spokesman Anatoly Proshin told Ukrainian news channel 112.ua.
Another spokesman for the military operation in the east, that has been battling a separatist rebellion since April, said rebels were responsible for the strike and that casualty figures were not yet known.
"Terrorists fired Grad missiles at a convoy ... we are waiting to hear how many died," he said.
Andrei Purgin, deputy prime minister of the self-proclaimed Donetsk People's Republic told Reuters rebel forces did not have the military capability to conduct such an attack.
"The Ukrainians themselves have bombed the road constantly with airplanes and Grads. It seems they've now killed more civilians, like they've been doing for months now. We don't have the ability to send Grads into that territory," he said by telephone.
The separatist conflict erupted in April after Russia annexed Ukraine's Crimean peninsula following the ousting of a Moscow-backed president in the capital Kiev.
Separatists occupied key buildings in towns across the Russian-speaking east, declaring 'people's republics' and saying they wanted to join Russia. (Reporting by Natalia Zinets, Thomas Grove; Writing by Alessandra Prentice; Editing by Andrew Heavens)