Ukraine on Saturday accused Russian forces of gunning down 24 civilians, including 13 children, in an attack on a road convoy near the recently recaptured town of Kupiansk.

On Friday, Ukrainian troops had shown AFP reporters a group of vehicles riddled with bullet holes and several corpses in civilian clothes outside the village of Kyrylivka, a short distance east of Kupiansk.

A day later, Kharkiv regional governor Oleg Synegubov put the preliminary death toll at 24 including, he said, a pregnant woman and 13 children, alleging: "Russians fired on the civilians at close range."

In an online video shared by official channels, a senior paramedic from the Kupiansk Department of Emergency Medical Care confirmed these numbers.

The Kharkiv region prosecutor's office said: "On September 30 prosecutors, investigators of the Security Service and the police discovered a convoy of seven cars that had been shot.

"The car queue was shot by the Russian army on September 25, when civilians were trying to evacuate," the statement alleged.

"Two cars have burnt completely with children and their parents inside -- they burnt alive," it said.

Ukrainian officials have launched an investigation and allege that the Russian forces that were driven out of Kyrylivka last Sunday were behind the war's latest apparent massacre.

"The occupiers are being defeated on the battlefield and desperately respond by killing civilians," said the head of the Security Service of Ukraine, Vasyl Malyuk.

"We will find everyone and hold them accountable for committed atrocities," he said. "Several people had managed to escape from Hell. Now they're testifying to the investigators."

The spokesman for the Kharkiv region prosecutor's office Dmytro Chubenko said during a TV interview that the massacre site's "grey area" frontline location was hampering the probe.

"There is no permanent control of the Ukrainian military over this territory," he said.

"It is periodically either under our control or under the control of Russia, so we cannot fully carry out investigative actions."

On Wednesday, an official from the Kremlin proxy administration in the occupied region of Lugansk had accused Kyiv's forces of firing on a civilian convoy, without providing evidence.

On Friday, AFP reporters counted the bodies of at least 11 civilians in the wrecked cars, some burned beyond recognition.

Ukrainian troops operating nearby said they had found the dead on an exposed section of road alongside a rail track, as defeated Russian forces retreated from the area.

- Bullet holes -

On Friday the bodies remained where they had died in and around the vehicles on a road running southeast from Kyrylivka 70 kilometres (43 miles) east of Kharkiv, in the direction of the Russian retreat.

A small van or minibus was completely burnt out, with the carbonised remains of four people within. At least one of these corpses appeared small enough to have been a child.

"The vehicles were moving all together as a civilian convoy. We can see that there is almost no distance between the vehicles," said "Filya", a Ukrainian soldier who guided AFP to the scene.

"And obviously they were attacked by gunfire because you can see the bullet holes ... One car was set on fire and it's impossible to recognise the people without an expert," he said.

Russia's February invasion allowed Moscow to seize a swathe of northern and eastern Ukraine, but this month a lightning Ukrainian counteroffensive in the Kharkiv region has driven them back.

Russian occupations forces have been accused of brutalising and murdering civilians several times during the seven-month-old war, and have hit cities with deadly missile strikes.