Earlier, a rebel source told the BBC no-one had survived an attack on a convoy of refugees in the Luhansk area on Monday.
Ukraine has accused pro-Russian rebels of attacking the convoy but they have denied involvement.
Meanwhile, heavy fighting is reported in the centre of Luhansk itself.
The government in Kiev said on Tuesday that street battles were taking place and an interior ministry aide told the Interfax Ukraine agency that the military was recapturing the city “block by block”.
The city centre was reported to have been shelled on Monday night.
Tens of thousands of civilians have fled Luhansk, one of two major cities held by pro-Russian rebels, as Ukrainian government forces advance. The city is suffering acute shortages of water, food and electricity.
Clashes have also prevented the recovery of bodies from the scene of Monday’s attack, on a road between Novosvitlivka and Khryashchuvatye.
A Ukrainian military spokesman, Andriy Lysenko, said that by 19:00 (16:00 GMT) on Monday, 15 bodies had been retrieved.
He had earlier said dozens of civilians had been killed, including women and children, when mortar rounds and Grad rockets had been fired at the convoy of buses and cars.
“The convoy had white flags and was marked as civilian,” Mr Lysenko said.
Ukrainian authorities in Kiev say the convoy was hit by Grad rockets and mortar rounds. However, a rebel source told the BBC that everyone in the convoy was killed when two buses carrying wounded civilians came under fire from foreign mercenaries fighting for the government.
The defence ministry in Kiev has posted a video which it says shows survivors describing what happened.
One man describes a car being “torn to bits” by a mortar round while another says his mother-in-law died instantly and his daughter bled to death.
Later in the defence ministry video, a soldier says two Zil lorries carrying refugees were hit by two mortar rounds. One was damaged and the other completely destroyed, he says.
Establishing who attacked the convoy will be difficult as both sides have supplies of the Russian-made Grad rockets.
The separatists have lost control of several small towns in the Luhansk and Donetsk regions in recent days.
Government forces say they have taken control of most of the town of Ilovaisk in Donetsk.
But a prominent commander, Semen Semenchenko, suffered shrapnel wounds when his unit came under mortar attack in the town. He is considered one of the most popular of the volunteer commanders in Ukraine.
More than 2,000 civilians and combatants have been killed since mid-April, when Ukraine’s government sent troops to put down the rebel uprising in the east.
Separatists in Luhansk and Donetsk declared independence from Kiev and proclaimed their own people’s republics after Russia annexed the Crimean peninsula from Ukraine in March.