He said the town of Borodyanka may have suffered more than others.
Horrific images of bodies in streets in towns like Bucha have generated shock and condemnation worldwide.
Russia denies killing civilians, and without evidence says Ukraine staged the scenes.
But the images prompted US President Joe Biden to call for his Russian counterpart Vladimir Putin to be tried for war crimes.
His intervention follows the publication of new satellite photos by the earth observation company Maxar, which show bodies lining Bucha’s streets during its occupation by Russian forces.