KIEV, Ukraine — Ukraine accused pro-Russia separatists of killing dozens of civilians in an attack early Monday on a convoy fleeing a besieged rebel-held city. The rebels denied any attack took place, while the U.S. confirmed the shelling of the convoy but said it did not know who was responsible.

The refugees were attacked with Grad rockets and other weapons imported from Russia as their convoy traveled between the towns of Khryashchuvate and Novosvitlivka, Col. Andriy Lysenko, a spokesman for Ukraine’s National Security Council, told reporters. The towns lie on the main road leading into Russia from the rebel-held city of Luhansk.

“Many people were killed, among them women and children,” 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.”

Oleksiy Dmytrashkivsky, a spokesman for the Ukrainian government’s military operation in the east, later told The Associated Press that 15 bodies had been recovered from the smoldering vehicles and servicemen were collecting the body parts of at least 10 more people.

Donetsk rebel chief Alexander Zakharchenko insisted that no such attack had taken place.

His deputy, Andrei Purgin, said he had no information about an attack and insisted it was not by his forces.

“If someone was killed, it wasn’t us but the Ukrainian military,” Purgin told the AP.

The road where Ukraine said Monday’s attack took place has been targeted previously by government forces, Purgin said.

It’s also same road to Luhansk that would likely be the route taken by a controversial Russian aid convoy if Ukraine allows it into the country.

The war zone in eastern Ukraine is effectively off limits for journalists and lacks power in many places, limiting citizens from easily providing their own reports. All this makes independent verification impossible.

The United States condemned the attack, but said it was unable to confirm who was responsible.

“We strongly condemn the shelling and rocketing of a convoy that was bearing internally displaced persons in Luhansk and express our condolences to the families of the victims,” U.S. State Department spokeswoman Marie Harf told reporters in Washington.

“All sides must take every precaution to protect innocent lives. We are unable to confirm reports of who was responsible for the shelling and rocketing.”

Fighting between government troops and pro-Russian separatists in eastern Ukraine has forced nearly 344,000 people to flee their homes, according to the United Nations, a number that has only grown as the humanitarian situation in rebel-held cities deteriorates. Running water and electricity have either been cut off completely in cities like Luhansk or are getting more limited by the day.

The U.N. Office for the Coordination of Humanitarian Affairs said more than 22,000 people fled the main rebel-held city of Donetsk last week compared to 6,200 the week before. And city officials have released even higher numbers: Donetsk has seen at least 300,000 of its pre-war population of 1 million leave their homes, while Luhansk has only 250,000 of its 420,000 people left, local authorities say.

Residents of Luhansk have had no running water, electrical power or phone connections for 16 days, and basic food is in short supply, leading to long lines outside shops as fighting continues in and around the city.

In Donetsk, the largest city in rebel hands, several houses were hit by artillery fire over the weekend in the Budyonovsky district, which stands next to a rebel encampment.