Dozens of people, including women and children, were killed fleeing fighting in eastern Ukraine yesterday 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.
"The rebels were expecting the convoy and destroyed it entirely," military spokesman Andriy Lysenko said. "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 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 get themselves out," military spokesman Anatoly Proshin said.
Describing the attack as a "bloody crime", Lysenko said: "A lot of people have been killed including women and children. The number of the dead is being established."
A rebel leader denied his forces had the capability to conduct such an attack, and accused Kiev forces of regularly attacking the area and also using Russian-made Grad missiles.
"The Ukrainians themselves have bombed the road constantly with airplanes and [missiles]. 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.
Another rebel spokesman denied any civilian convoy had been struck.
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 to address claims by the rebels to have received new stocks of heavy Russian equipment and 1,200 trained Russian fighters.
Referring to the heavy loss among government forces, he said: "Today we need to regroup our forces to defend our territory and continue offensive actions by the army."
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 with the Ukrainian, German and French foreign ministers on a ceasefire or a political solution.
Russia says it would like a ceasefire to allow aid to get to people trapped by the fighting. A 280-truck convoy sent by Russia and carrying tonnes of humanitarian aid has been stalled at the Ukrainian border since last week, as Kiev has insisted on formalities so it can be properly distributed by the Red Cross.
Ukrainian Foreign Minister Pavlo Klimkin said: "Russia must close the border and stop shelling. If you have mercenaries and weaponry coming through the border from [Russia] how can you reach a ceasefire? It's not about terminology or conditions. It's about substance."
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 such offences as treason, desertion, espionage and attempts on the lives of the leadership.
"Introducing the death penalty is not revenge, it is the highest degree of social protection," the senior rebel leader, Vladimir Antyufeyev, said.