Seventeen people are believed to have been beheaded by the Taliban, after late-night music and dance party for young Afghan men and women.
Some 15 men and two women were found beheaded in Afghanistan's southern Helmand province on Monday, although the Taliban has not yet claimed responsibility.
District governor Nimatullah Khan told Reuters that the bodies were found in a house near the Musa Qala district, 46 miles from the provincial capital Lashkar Gah.
Helmand governor spokesman Daud Ahmadi said a team had been sent to the site of beheadings to investigate. He said all of the victims had had their heads cut off, but they may have been shot first. The district is entirely controlled by the Taliban.
Mixed-sex parties, especially those which involved dancing and music, are highly controversial and are usually held in secret.
For the Taliban, flirting, open displays of affection and the mixing of men and women are vehemently condemned.
In June, Taliban gunmen stormed a high-end hotel on Qarga Lake demanding to know where the "prostitutes and pimps" were, and killed 20 people.
The Taliban said the venue was used for "wild parties".
Haji Musa Khan, a tribal elder in the neighbouring district of Musa Qala, told AP the region had seen a surge in killings.
"We had three people beheaded during the month of Ramadan. Another person, the son of a tribal elder, was beheaded recently."
Under Taliban rule, women were denied education, employment, freedom of movement and political participation and representation.
They were excluded from public life and prohibited from studying, working or leaving the house unless chaperoned by a male relative.
Foreign Office Minister Alistair Burt said he was "appalled" by the reports, adding: “The facts are still being established but early indications are that the Taliban were responsible. We condemn acts of extreme violence like this in the strongest terms.
“This incident, alongside the attack today on a Helmand checkpoint where ten Afghan soldiers were killed, underlines the continuing importance of our work to strengthen the capacity of the Afghan security authorities."