BAGHDAD, (Reuters) – The U.S. military said on Wednesday al Qaeda was the “prime suspect” in suicide bombings overnight on an ancient minority sect that Iraqi officials said killed more than 175 people in northern Iraq.

Rescue workers searched for bodies in the rubble of dozens of buildings destroyed in up to five simultaneous car bomb attacks. The attackers, driving fuel tankers, struck densely populated residential areas west of the city of Mosul that are home to members of the Yazidi sect, whose followers are considered infidels by Sunni Islamist militant groups.

The U.S. military said it was too early to say who was responsible, but the scale and apparently coordinated nature meant the attack carried the hallmarks of Sunni Islamist al Qaeda. The United States has condemned the attack as barbaric. “We’re looking at al Qaeda as the prime suspect,” said U.S. military spokesman Lieutenant-Colonel Christopher Garver. The death toll was the highest since November when six cars bombs in different parts of Baghdad’s sprawling Shi’ite slum of Sadr City killed 200 people and wounded 250, while multiple car bombs killed 191 around Baghdad in April.

The U.S. military has launched a major new offensive in Iraq in a bid to thwart such attacks by al Qaeda and Shi’ite militias ahead of a progress report on the U.S. military strategy in Iraq that is due to be presented to the U.S. Congress in September.

In the aftermath of the blast, authorities imposed a total curfew in the Sinjar area, which is close to the Syrian border.

Sinjar district mayor Dakheel Qassim Hasoun said only people and vehicles involved in rescue efforts would be allowed to move through the area. He said it would be impossible to establish a final death toll any time soon because many bodies were still buried in the rubble of up to 30 houses destroyed in the blasts.

Lieutenant-Colonel Mike Donnelly, U.S. military spokesman for northern Iraq, said U.S. forces were assisting Iraqi emergency agencies as they sifted through the rubble and were providing logistical, security and medical support.

Iraqi army captain Mohammad al-Jaad put the death from the attack by at least three suicide bombers driving fuel tankers at 175, with 200 wounded. Hasoun said the death toll could go as high as 200.

Iraqi authorities said the death toll was so high because most of the destroyed houses, all tightly packed in three Yazidi residential compounds, were made of mud that were shattered by the force of the attack. “It is going to be difficult to get a full death toll because of the nature of the buildings,” Garver said.

The U.S. military said five vehicle-borne bombs had been detonated in Yazidi residential compounds in the villages of Kahtaniya and al-Jazeera. Jaad said the village of Tal Uzair was also hit.

Yazidis are members of a pre-Islamic Kurdish sect who live in northern Iraq and Syria. Sunni militants have targeted Yazidis in recent months by kidnapping and killing them.

Yazidis in Iraq say they have often faced discrimination because the chief angel they venerate as a manifestation of God is often identified as the fallen angel Satan in biblical terminology.