The mayor of Bucha said on Saturday that nearly 300 people have been buried in a mass grave in the commuter town outside Kyiv.

He said it happened after Ukrainian army retook control of the key town from Russian forces.

"In Bucha, we have already buried 280 people in mass graves," mayor Anatoly Fedoruk was quoted by NDTV which sourced the news from AFP.

Explaining the extent of the brutality, he said the heavily destroyed town's streets are littered with corpses.

At least 20 bodies, all men in civilian clothes, were seen lying in a single street in Bucha on Saturday.

According to Fedoruk, all these people were shot, killed, in the back of the head.

The victims included men, women and also a 14-year-old boy who was among the dead.

Several bodies had white bandages on them in a sign that "they were unarmed," the mayor said.

There were cars in the streets with "entire families killed: children, women, grandmothers, men," he added.

Giving an account, Fedoruk 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 adding it was not possible to tell how many civilians had been killed during fighting with Russian forces.