The accounts of atrocities were not limited to the Islamic State, a Sunni militant group also known as ISIS or ISIL. Iraqi government security forces and militias associated with them have also committed “gross violations” of international law, abducting civilians and hanging their bodies from lampposts in Baquba, summarily killing captured Islamic State fighters and launching airstrikes that resulted in “significant” civilian deaths and injuries, according to the report.

Image The aftermath of a bombing that killed 14 people and wounded at least 43 on Thursday in Baghdad. Credit... Ahmad Al-Rubaye/Agence France-Presse — Getty Images

But as the Islamic State seeks to expand its control of parts of northern Iraq, its campaign of terror has shocked the world, officials said.

“The array of violations and abuses perpetrated by ISIL and associated armed groups is staggering, and many of their acts may amount to war crimes or crimes against humanity,” Zeid Ra’ad Zeid al-Hussein, the United Nations human rights chief, said in a statement that urged the new Iraqi government to accept the jurisdiction of the International Criminal Court to investigate whether such crimes had been committed.

Drawing on the testimony of survivors and witnesses, the agencies report multiple mass killings by Islamic extremists. Spotlighting the almost random brutality of the Islamic State, the report notes that three Sunni women were killed after sentencing by an Islamic State-appointed court in Mosul on Sept. 5; 40 men were killed on the orders of the same court on Sept. 7; and two women were shot in the back of the head on Sept. 9.

In addition to the killing of civilians, Islamic State gunmen have claimed responsibility for killing 1,500 soldiers and security forces at a military base captured in June, the United Nations said. The dead include 42 soldiers captured south of Tikrit in July and 18 police officers whose bodies were left by the roadside nearby, the report said.