"In the Kupiansk district, a shot-at convoy of cars containing civilians was found. According to preliminary data, 20 people died in the cars," governor Oleg Synegubov said on the Telegram messenger site.

Russians "attacked civilians who tried to escape the shelling. This is cruelty that has no justification," Synegubov added.

On Friday AFP reporters saw the bodies of at least 11 civilians, who died on an exposed section of road as defeated Russian forces retreated from Kupiansk.

The Ukrainian troops who found the group of destroyed civilian cars with dead and in some cases burned bodies, said they believed Russian troops had attacked a civilian convoy.

The bodies remained where they died in and around the six vehicles on a road from the village of Kyrylivka, some 70 kilometres (43 miles) east of Kharkiv.

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.

Russia's February invasion left its forces in command of a swathe of northern and eastern Ukraine, but this month a lightning counteroffensive in the Kharkiv region has driven them back.

Russian forces have been accused of brutalising and murdering civilians in occupied areas several times during the seven-month-old war.