BAGHDAD -- A recent spike in civilian casualties in Mosul suggests the U.S.-led coalition is not taking adequate precautions to prevent civilian deaths as it battles Islamic State of Iraq and Syria (ISIS) militants alongside Iraqi ground forces, Amnesty International said on Tuesday.

The human rights group’s report followed acknowledgement from the coalition that the U.S. military was behind a March 17 strike in a western Mosul neighborhood that residents said killed more than a hundred civilians.

Amnesty’s report also cited a second strike on Saturday that it said killed “up to 150 people.” U.S. officials have opened an investigation to verify the credibility of the claims of civilian casualties.

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The Amnesty report said evidence gathered on the ground in Mosul “points to an alarming pattern of U.S.-led coalition airstrikes which have destroyed whole houses with entire families inside.”

It said any failure to take precautions to prevent civilian casualties would be “in flagrant violation of international humanitarian law.”

In Baghdad, visiting U.S. army chief of staff, Gen. Mark A. Milley, said on Monday that what caused the explosion was still unknown and added that “some degree of certainty will be known in the coming days following the investigation.”

“It is very possible that Daesh blew up that building to blame it on the collation in order to cause a delay in the offensive into Mosul and cause a delay in the use of collation airstrikes, that is very possible,” Milley told reporters after meetings at the Iraqi Defense Ministry.

Daesh is an Arabic language acronym for ISIS.

“And it is possible the collation airstrike did it,” he added.

U.S. Central Command spokesman Col. John Thomas said Monday that the U.S. military is looking at 700 different video feeds, involving about as many munitions, over ten days pertaining to the strikes in the region. The review is also taking into account Iraqi claims of booby traps or vehicle-borne IEDs in the vicinity of the Mosul apartment buildings that were flattened.

There is, Thomas said, “intriguing information” about secondary explosions. He also said the U.S. is looking to get “folks on the ground” to assist in the review.

Hours after Amnesty criticized the offensive around Mosul, United Nations human rights chief Zeid Ra’ad Al Hussein said at least 307 civilians had been killed and 273 wounded in western Mosul since February 17. He said ISIS was corralling people into booby-trapped buildings to be used as human shields, and that he deplored the “massive loss of civilian lives.”

“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,” he said, according to the Reuters news agency.

Iraqi forces began the assault on ISIS-held Mosul in October, after months of preparation and buildup. In January, Iraq declared the eastern half of Mosul - the Tigris River divides the city into an eastern and western sector - “fully liberated.” Iraqi government forces are now battling to retake the city’s western half.

Civilians, humanitarian groups and monitoring officials have repeatedly warned of the possibility of increased civilian casualties in western Mosul due to the higher density of the population there and the increased reliance on airstrikes and artillery. Faced with their toughest fight against ISIS yet, Iraqi and coalition forces have increasingly turned to airstrikes and artillery to clear and hold territory in Mosul’s west.

Unlike its previous battles against ISIS in urban settings in Iraq, the government made the decision to instruct Mosul civilians to remain in their homes. In the battles for Fallujah and Ramadi, those cities were entirely emptied of their civilian population while Iraqi forces fought to push out ISIS. In Mosul, the Iraqi government said it asked civilians to remain in place to prevent large-scale displacement.

When the operation to retake Mosul was launched, more than a million people were estimated to still be living in the city, Iraq’s second-largest. Today, the United Nations estimates about 400,000 people remain trapped in ISIS-held neighborhoods in western Mosul.

Amnesty International’s report quoted survivors and eyewitnesses of airstrikes that have killed civilians as saying that “they did not try to flee as the battle got underway because they received repeated instructions from the Iraqi authorities to remain in their homes.”