The Pentagon said on Monday it could not independently confirm the accounts of atrocities but had no reason to dispute them.
A Reuters reporter saw several dead civilians in the town, including one with his hands bound behind him. Local residents said hundreds of civilians had been killed.
Bucha's deputy mayor, Taras Shapravskyi, said 50 of the dead residents, found after Russian forces withdrew from the city late last week, were the victims of extra-judicial killings carried out by Russian troops. Ukrainian officials, including President Volodymyr Zelenskiy, have accused Moscow of war crimes.
"These are war crimes and will be recognised by the world as genocide," Zelenskiy said, speaking on television from Bucha, wearing body armour and surrounded by military personnel.