According to Reuters, dozens of people, including women and children, were killed as they fled fighting in eastern Ukraine on Monday (August 18) when their convoy of buses was hit by rocket fire, military spokesmen said.
Ukraine accused pro-Russian rebels of targeting the convoy, which it said was bearing white flags when it was hit near the eastern city of Luhansk. The separatists denied responsibility for the attack and one rebel leader suggested the incident might never have taken place.
"The separatists were expecting the convoy and destroyed it entirely," military spokesman Andriy Lysenko told journalists. "We haven't been able to count the number of victims... dozens (were killed)."
The convoy had been in an area of fierce fighting between government forces and the separatists when it came under fire from rebel Grad and mortar launchers, the spokesmen said.
"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 escape," military spokesman Anatoly Proshin told a Ukrainian news channel.
COMMENT: A rebel leader denied his forces had the military capability to conduct such an assault, and accused Kiev forces of regularly attacking the area and also using Russian-made Grad missiles.
The Kiev military reported new successes overnight, building on a weekend breakthrough when troops raised the national flag in Luhansk, a city held by pro-Russian separatists since fighting began in April.
Troops blockaded or recaptured rebel-held positions after international talks in Berlin failed to reach agreement on a ceasefire. Nine soldiers were killed.
Western sanctions against Moscow have failed to stem what NATO says is 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 Foreign Minister Sergei Lavrov said all issues around a humanitarian convoy sent by Moscow to relieve needy areas of eastern Ukraine had been resolved, but no progress had been made in his talks in Berlin on Sunday with the Ukrainian, German and French foreign ministers on a ceasefire or a political solution.
Ukrainian Foreign Minister Pavlo Klimkin, who was at the Berlin talks, said in comments posted by the ministry on Twitter: "Russia must close the border and stop shelling. If you have mercenaries and weaponry coming through the border from the Russian federation, how can you reach a ceasefire?"
In a further sign that the rebel leadership may be facing deep problems in its ranks, it said it was setting up military tribunals and bringing in the death penalty for a string of offences including desertion, espionage, attempts on the lives of the leadership and sabotage.
The possibility the rebels might be facing a rout presents Russian President Vladimir Putin, who boosted their ambitions by speaking of the creation of a "New Russia" in eastern Ukraine, with a difficult choice.
If he allows their defeat, he risks losing face before the "hawks" at home and the Russian people who largely applauded Moscow's annexation of Crimea in March. But maintaining pressure on Kiev's pro-Western leaders through further support for the rebels risks wider economic sanctions from the United States and European Union.
Western sanctions have targeted Russia's financial and energy sectors as well as dozens of people close to Putin, and Russia has retaliated by banning a wide range of US and EU food imports.
Vedomosti daily newspaper said Moscow might ban imports of cars, among other things, if the United States and the EU take additional action against it.
The separatist conflict erupted after Russia seized the Crimean peninsula in March, following the ousting of a Moscow-backed Ukrainian president. Separatists occupied key buildings in towns across the Russian-speaking east, declaring "people's republics" and saying they wanted to join Russia.
The United Nations said this month that an estimated 2,086 people, including civilians and combatants, had been killed in the conflict. The death toll has nearly doubled since the end of July, when Ukrainian forces stepped up their offensive as they gained more ground against the rebels.
A military spokesman in Kiev said government forces had pressed the separatists in overnight fighting, encircling the rebel-held town of Horlivka between Luhansk and Donetsk, and taking control of smaller settlements in eastern Ukraine.
The military said it suspected the rebels had used a powerful Russian-made Uragan missile system for the first time southeast of Donetsk near the village of Novokaterinivka.