The Taliban murdered 13 ethnic Hazara people including a teenage girl, according to a prominent rights group.
Amnesty International said it found evidence the victims were massacred in Daykundi province in August.
Nine were former government soldiers who had surrendered to the Taliban, Amnesty said, adding that the killings appeared to be a war crime.
The Taliban denied the allegation, telling the BBC that the Amnesty report only showed "one side" of the story.
The Hazara community is Afghanistan's third largest ethnic group. They mainly practise Shia Islam and have faced long-term discrimination and persecution in predominantly Sunni Afghanistan and Pakistan. This is the second time the Taliban have been accused of killing Hazaras since the group swept to power in August.
Two other victims of the alleged killings in Daykundi province were civilians, Amnesty said, including a 17-year-old girl reportedly shot when the Taliban opened fire on a crowd of soldiers' families.
The civilians were killed as they attempted to flee, Amnesty said in the report published on Tuesday.
"These cold-blooded executions are further proof that the Taliban are committing the same horrific abuses they were notorious for during their previous rule of Afghanistan," said Agnès Callamard, secretary-general of Amnesty International.
"The Taliban say they are not targeting former employees of the previous government, but these killings contradict such claims," she said.
A previous Amnesty report, released in August, said the Taliban had "massacred" nine members of the Hazara minority in Ghazni province in July.
The Taliban's interior ministry spokesman, Qari Saeed Khosti told the BBC: "This report is one sided and we call on all international organisations to come and conduct a proper investigation in the field.
"This is not an acceptable conclusion and is free of transparency."
According to the Amnesty report, about 300 Taliban fighters travelled on 30 August to an area near Dahani Qul village, where members of the former government forces were staying with their families.
The former Afghan security forces members and their families attempted to flee, but the Taliban caught up with them and opened fire, the report said.
One former soldier fired back, killing a Taliban fighter and wounding another, Amnesty said, and two other former government soldiers were killed in the ensuing crossfire.
Nine other former soldiers then surrendered, according to the report, but the Taliban "promptly took them to a nearby river basin and executed them".