At least 307 civilians have been killed and 273 wounded in western Mosul since Feb. 17, as ISIS fighters herd people into booby-trapped buildings as human shields and fire on those who flee, the United Nations human rights chief said on Tuesday.

"This is an enemy that ruthlessly exploits civilians to serve its own ends, and clearly has not even the faintest qualm about deliberately placing them in danger," UN High Commissioner for Human Rights Zeid Ra'ad al-Hussein said in a statement.

"It is vital that the Iraqi Security Forces and their coalition partners avoid this trap," he said, while calling for them to conduct transparent investigations into deadly incidents involving their forces.

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

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Smoke rises from clashes during a battle between Iraqi forces and ISIS militants in Mosul on Monday. At least 307 civilians have been killed in the city since Feb. 17. (Khalid al Mousily/Reuters)

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

"The fact that Iraqi authorities repeatedly advised civilians to remain at home instead of fleeing the area, indicates that coalition forces should have known that these strikes were likely to result in a significant number of civilian casualties," said Amnesty's Donatella Rovera, who carried out field investigations in Mosul. 

Investigation into March 17 blast

Investigators are in Mosul to determine whether a U.S.-led coalition strike or ISIS-rigged explosives caused a huge blast that destroyed buildings and killed more than 200 people on March 17, a U.S. military commander said.

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

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Displaced residents who fled their homes wait to be searched by Iraqi troops in western Mosul on Tuesday. Thousands have fled the city, but as many as half a million may be trapped inside. (Suhaib Salem/Reuters)

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 Shia-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 defence 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.

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A federal police military vehicle is seen during a battle between Iraqi forces and ISIS in Mosul on Monday. The U.S. is investigating civilian deaths in the city. (Khalid al Mousily/Reuters)

"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 ISIS. "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.

Half a million trapped

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.

The battle is focused now on the Old City where troops are moving on the Al Nuri mosque, where ISIS'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.