The U.S.-led coalition fighting the Islamic State in Iraq and Syria (ISIS) has killed at least 484 civilians since the start of the campaign, the coalition said in a monthly report released Friday.

The total casualty figure spiked 132 from April, largely due to the previously acknowledged 105 civilians who were killed after a U.S. airstrike in March set off explosives laid by ISIS in a building in Mosul, Iraq.

The coalition “takes all reports of civilian casualties seriously and assesses all reports as thoroughly as possible,” the statement said. “Although the coalition takes extraordinary efforts to strike military targets in a manner that minimizes the risk of civilian casualties, in some incidents casualties are unavoidable.”

Independent monitors and human rights groups say the Pentagon lowballs the number of civilian causalities. The prominent monitor Airwars places the number at 3,817.

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In addition to the Mosul building collapse, the coalition found 15 other reports of civilian casualties in January, February, March and April to be credible. The coalition defines a credible assessment as one that “more likely than not” resulted in a civilian causality.

Eleven of the credible assessments were described as happening near Mosul, in addition to the building collapse, underscoring the fierce fighting that continues as forces work to expel ISIS from its remaining neighborhoods. Those 11 reports accounted for 21 deaths, according to the findings.

Other credible reports were from Deir ez-Zor, Syria; Raqqa, Syria; al-Qaim, Iraq; and Tal Afar, Iraq, accounting for six deaths.

The coalition also said that 31 reports of civilian casualties in September, December, February, March and April were deemed noncredible, which means that there was “not sufficient information available” to determine that civilian casualties were more likely than not.

Finally, 38 other reports of civilian casualties are still being assessed, including one from April 2015 that is being reopened after new information was provided, the coalition said.

Advocates have expressed concern that civilian casualties are on the rise and suspect the Pentagon has changed its rules of engagement. The Pentagon has insisted that nothing has changed and said any increase is due to the phase of the war and ISIS's increased use of human shields.

In an interview on CBS last Sunday, Defense Secretary James Mattis said civilian casualties are a “fact of life” in war.

“Civilian casualties are a fact of life in this sort of situation,” he said. “We do everything humanly possible, consistent with military necessity, taking many chances to avoid civilian casualties at all costs.”