Dozens of civilians were killed when separatist rebels shelled a convoy of refugees trying to flee war-torn eastern Ukraine, a top Ukrainian official said.
Colonel Andriy Lysenko, a spokesman for Ukraine's National Security Council, told reporters the barrage took place between the towns of Khryashchuvate and Novosvitlivka, which lie on the main road leading to Russia from the besieged rebel-held city of Luhansk.
"Many people were killed, among them women and children," Col Lysenko said. "We are not able to count the death toll at this point."
When asked about a rough estimate of deaths, he said "dozens".
Yet Donetsk rebel chief Alexander Zakharchenko later told reporters that no such attack had taken place. His deputy, Andrei Purgin, said he had no information about any attack but insisted it was not by his forces.
"If someone was killed, it wasn't us but the Ukrainian military," Purgin told the Associated Press.
Purgin said the road cited by the Ukrainian government for Monday's alleged attack had been targeted previously by government forces. That same road to Luhansk would probably be the one taken by a disputed Russian aid convoy if Ukraine allows it into the country.
It was not possible to immediately reconcile the differing statements, since the warzone is very dangerous for journalists and a lack of power there limits independent reports from civilians.
Fighting across eastern Ukraine has forced nearly 344,000 people to flee their homes, according to UN figures released on Friday. The UN says about 155,800 have left for other places inside Ukraine while 188,000 more have crossed into Russia.
The flow of refugees only seems to be growing. The UN Office for the Co-ordination of Humanitarian Affairs said more than 22,000 people fled the main rebel-held city of Donetsk last week compared with 6,200 the week before.
City officials have released even higher numbers. Donetsk has seen at least 300,000 of its pre-war population of one million leave their homes, while Luhansk has only 250,000 of its 420,000 people left, local authorities say.
Residents in Luhansk have had no running water, power or phone connections for 16 days. Basic foods are in short supply, leading to long lines outside shops, city hall said, adding that fighting continues in and around the city.
The International Committee of the Red Cross (ICRC), which is expected to take responsibility for the Russian aid convoy when it enters Ukraine, has demanded security guarantees from all sides, including the rebels, for the mission into eastern Ukraine. As of Monday, there was no indication that the guarantees had been given.
The humanitarian aid convoy of over 200 trucks from Russia has been watched with suspicion by Ukraine and the West, especially since Ukrainian forces have been winning back significant territory from the rebels in the last few weeks. They suggest it could be used by Russia to send help to the separatists - or to delay the government's advances with a timely ceasefire.
Russia's foreign minister, meanwhile, said he expects the Russian aid mission to enter Ukraine in the near future.
Speaking at a news conference in Berlin, where he met a day earlier with his counterparts from Ukraine, France and Germany, Sergey Lavrov said all questions regarding the mission had been answered and that an agreement had been reached with Ukraine and the ICRC.
It was not clear if Mr Lavrov was referring to the security guarantees.
The West and Ukraine have accused Moscow of supplying rebels with arms, including sophisticated weapons. Zakharchenko, the rebel leader, once again brushed off these claims, saying the weapons the separatists were using had been seized from the Ukrainian army.
The fighting in eastern Ukraine began in mid-April, a month after Russia annexed Ukraine's southern peninsula of Crimea.