Almost 300 people have been buried in a mass grave in Bucha, a commuter town outside Ukraine's capital Kyiv, its mayor told AFP Saturday after the Ukrainian army retook control of the key town from Russia.

"In Bucha, we have already buried 280 people in mass graves," mayor Anatoly Fedoruk told AFP by phone. He said the heavily destroyed town's streets are littered with corpses.

AFP saw at least 20 bodies -- men in civilian clothes -- lying in a single street in Bucha on Saturday. 

"All these people were shot, killed, in the back of the head," Fedoruk said. 

He said the victims were men and women, and that he had seen a 14-year-old boy among the dead. 

Many of the bodies had white bandages on them "to show that they were unarmed," he said. 

The town still had cars in the streets with "entire families killed: children, women, grandmothers, men," he added.  

The corpses were still in the streets because sappers have not worked there yet, Fedoruk said.  

He claimed some of the victims had tried to cross the Buchanka river to Ukrainian-controlled territory and had been killed. 

"These are the consequences of Russian occupation," he said.   

He said it was not possible to tell how many civilians had been killed during fighting with Russian forces. 

Authorities will clear the corpses after sappers give them the green light in "three or four days", he said. 

Ukrainian forces regained control of Bucha this week. The town had been inaccessible for almost a month as it was held by Russian forces.