Ukrainian officials said dozens of civilians were killed on Monday, August 18, in an attack by pro-Russian rebels on a convoy trying to escape fighting in the country's east.
A rebel spokesman denied his forces were involved in the attack.
Ukraine accused the rebels of firing rockets and mortars on a convoy of buses carrying refugees who were trying to flee the region's intense fighting near the separatist-held city of Luhansk.
Dozens of people, including women and children, were killed in the shelling on Monday, said Andriy Lysenko, a spokesman for Ukraine's national security council.
“The rebels were expecting the convoy and destroyed it entirely. We haven't been able to count the number of victims ... dozens [were killed],” spokesman Andriy Lysenko said, adding that he was unable to provide exact casualty figures.
Lysenko also said the rebels used "Grad" systems and mine-launchers imported from Russia to target the civilian convoy, which it said was bearing white flags.
The strike took place Monday morning between the towns of Khrashchuvate and Svitlivka, which lie on the main road leading from the besieged eastern city of Luhansk.
The allegations came after a five-hour meeting between the foreign ministers of Ukraine, Russia, France and Germany broke up without agreement on how to end more than four months of conflict that has killed over 2,100 people and left the region facing a humanitarian catastrophe.
Rebels blame Ukraine
A rebel leader denied his forces had the military capability to conduct such an attack, and accused Kyiv forces of regularly attacking the area and also using Russian-made Grad missiles.
“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,” said Andrei Purgin, deputy prime minister of the self-proclaimed Donetsk People's Republic.
There were no immediate further details.
That road is likely the one that a convoy of Russian humanitarian aid would take if Ukraine allows it into the country.
Reports of fresh successes by the Kyiv military followed a breakthrough for government forces at the weekend when troops raised the national flag in Luhansk, a city held by the pro-Russian separatists since the onset of the conflict in April.
However, nine Ukrainian troops were killed there in overnight fighting, a military spokesman said.
Western sanctions against Moscow have failed to stem what NATO calls a steady supply of military equipment and men sent from Russia to help the rebels. Russia denies sending support, saying the rebels have seized equipment from the Ukrainians.
President Petro Poroshenko called on his top security advisers on Monday to address claims by the rebels to have received new stocks of heavy Russian military equipment and 1,200 trained Russian fighters.
Russian aid convoy
Meanwhile, a massive aid convoy sent from Moscow was still waiting to be checked near Ukraine's restive border as talks dragged on about allowing them to cross into rebel-held territory.
On Sunday, Russian Foreign Minister Sergei Lavrov said “all questions” relating to Russia sending the humanitarian convoy to Ukraine had been addressed.
However, the International Committee of the Red Cross, which is to take responsibility for the aid convoy when it enters Ukraine, has demanded security guarantees from all sides, including the rebels, for the mission.
"We are still waiting for security guarantees for the convoy," said Galina Balzamova, spokeswoman for the ICRC.
As of midday, there was no indication that the guarantees had been given.
The convoy has been parked for days in Russia near the border amid objections from Kyiv, which believes the convoy could be a Trojan Horse for Russia to get weapons to the rebels - a notion that Moscow has dismissed as absurd.