By Isabel Coles

MOSUL, Iraq (Reuters) – Investigators are in the Iraqi city of Mosul to determine whether a U.S.-led coalition strike or Islamic State-rigged explosives caused a blast that destroyed buildings and may have killed more than 200 people, a U.S. military commander said.

Conflicting accounts have emerged since the March 17 explosion in al-Jadida district in west Mosul, where Iraqi forces backed by U.S.-led coalition air strikes are fighting to clear Islamic State militants from Iraq’s second city.

Iraq’s military command has blamed militants for rigging a building with explosives to cause civilian casualties, but some witnesses say it was collapsed by an air strike, burying many families under the rubble.

If confirmed, the toll would be one of the worst since the 2003 U.S.-led invasion, raising questions about civilian safety as Iraq’s Shi’ite-led government tries to avoid alienating Mosul’s mostly Sunni population.

U.S. Army chief of staff Gen. Mark Milley, after meeting Iraqi Prime Minister Haider al-Abadi and Iraq’s defense minister late on Monday, said there had been air strikes in the vicinity that day and on previous days but it was not clear they had caused the casualties.

“It is very possible that Daesh blew up that building to blame it on the coalition in order to cause a delay in the offensive on Mosul and cause a delay in the use of coalition air strikes,” Milley said, using an Arabic term for Islamic State.

“It is possible that a coalition air strike did it. We don’t know yet. There are investigators on the ground.”

A source close to Abadi’s office said the U.S. military delegation also called for more coordination among the Iraqi security force units on the ground and for consideration that thousands of civilians are stuck in their homes.

The United Nations rights chief said on Tuesday at least 307 civilians have been killed and 273 wounded in western Mosul since Feb. 17, saying Islamic State was herding residents into booby-trapped buildings as human shields and firing on those who tried to flee.

“This is an enemy that ruthlessly exploits civilians to serve its own ends,” U.N. High Commissioner for Human Rights Zeid Ra’ad al-Hussein said. “It is vital that the Iraqi security forces and their coalition partners avoid this trap.”

PRECAUTIONS

Iraqi forces have retaken eastern Mosul and are pushing through the west but have faced tough resistance in the densely populated districts around the Old City, where narrow streets and traditional homes force close-quarters fighting.

Iraqi forces fighting around the Old City tried to storm the al-Midan and Suq al Sha’areen districts, where Islamic State ran its religious police who carried out brutal punishments, such as crucifixion and public floggings, federal police commander Lt. Gen. Raed Shakir Jawdat told state al-Sabah newspaper.

Helicopters were strafing Islamic State targets around Al Nuri mosque, where Islamic State’s leader declared his caliphate nearly three years ago after militants took control of swaths of Iraq and Syria.

Thousands of civilians are fleeing the fighting, shelling and air strikes, but as many as half a million people may be trapped inside the city. Fleeing residents say they have been used as human shields by militants who shelter in their homes.

The Iraqi Observatory for Human Rights said since the campaign against western Mosul began in February, unconfirmed reports have said nearly 700 civilians have been killed by government and coalition air strikes or Islamic State action.

Rights group Amnesty International said the high civilian toll suggested U.S.-led coalition forces had failed to take adequate precautions to prevent civilian deaths.

The al-Jadida incident is far from clear. Witnesses on Sunday described horrific scenes of body parts strewn over rubble, residents trying desperately to pull out survivors and other people buried out of reach.

The Iraqi military’s figure of 61 bodies was lower than that given by local officials – a municipal official said on Saturday 240 bodies had been pulled from the rubble. A local lawmaker and two witnesses say a coalition air strike may have targeted a truck bomb, triggering a blast that collapsed buildings.

(Additional reporting by Ahmed Rasheed in Baghdad; Writing by Patrick Markey; Editing by Janet Lawrence)