A United Nations investigation published Sunday confirmed that Taliban and self-proclaimed Daesh insurgents jointly massacred dozens of people in Afghanistan earlier this month in an attack that "may amount to a war crime".

The body's mission in Afghanistan (UNAMA) said it had "verified allegations" of at least 36 deaths in the predominantly Shiite village of Mirzawalang in Sayad district of northern Sar-e Pul province.

"These killings, corroborated by multiple credible sources, constitute violations of international humanitarian law and may amount to war crimes," UNAMA said in its report. It added that more than half of the killings took place on Saturday 5 August when civilians tried to flee the village after militants had captured it following a battle with a government-backed militia.

Afghan officials claim that Taliban and Daesh fighters killed more than 50 villagers, including by beheadings, in a rare joint operation between the two insurgent groups. The U.N. investigation said that as many as 27 civilians were killed, including one woman, four teenage boys and 13 men over the age of 60.

Also among the dead were at least seven pro-government militia fighters, one local policeman, and an Afghan army soldier, it added. It was unable to confirm the beheading claims.

The U.N. investigation noted that a commander implicated in the raid had claimed allegiance to Daesh but concluded it was "not aware of any information supporting his links" to the wider Daesh group.

Taliban and Daesh fighters have regularly clashed in Afghanistan over the past two years but allegiances are occasionally fluid and security sources say they have teamed up in the past to strike Afghan forces in certain areas. The Taliban had earlier confirmed capturing Mirzawalang but said it did so alone. It has also denied allegations it had killed civilians.

Last week, Daesh claimed responsibility for killing 54 Shiites in Sar-e Pul in a statement released by its propaganda outlet Amaq.

The people of war-torn Afghanistan continue to bear the brunt of the grinding conflict with civilian deaths at their worst since records began, the United Nations said. The majority of the victims were killed by anti-government forces, including the resurgent Taliban and in attacks claimed by Daesh terrorist group, the report said, underscoring spiraling insecurity in the country nearly 16 years after the U.S. invasion. The U.N. has documented civilian casualties in the war-torn country since 2009.

The first six months of the year have seen a significant rise in the number of civilian lives lost in highly coordinated attacks involving more than one perpetrator, with 259 killed and 892 injured, a 15 percent increase on the same period last year. Many of those deaths happened in a single attack in Kabul in late May when a truck bomb exploded during the morning rush hour, killing more than 150 people and injuring hundreds.