A new report from a group of journalists and researchers says that hundreds of civilians have died during airstrikes by the U.S. and other nations fighting the Islamic State, a marked contrast to the Pentagon’s official admission of just two civilian deaths.

The report, from the nonprofit group Airwars, which tracks coalition airstrikes on Iraq and Syria, says that it has documented between 459 and 591 civilian deaths in 52 credible incidents. In one of the worst cases, in Al Bab, Syria, a U.S. strike on a local Islamic State headquarters being used as a jail killed up to 58 non-combatants, including women and teenagers.

Next Saturday marks the first anniversary of the United States’ bombing campaign against Islamic State militants in Iraq. Over the past year, a U.S.-led coalition including Canada, France, Australia, Saudi Arabia, and other European and Gulf states has carried out over 5,800 airstrikes against the group in Iraq and Syria.

Airwars notes that the coalition officially estimates that between 10,000 and 13,000 Islamic State fighters have been killed, and more than 2,000 buildings destroyed.

Given those numbers, the report’s author, journalist Chris Woods, notes “a worrying gulf” between the official accounting for the war and consensus among monitoring groups operating in Iraq and Syria, which estimate hundreds of civilian deaths. Airwars has published its research into instances of civilian casualties along with the report, including photographs and the names of more than 260 victims.

Centcom has only publicly admitted to two civilian deaths from the campaign: two girls “likely” killed in Syria last November.

Woods notes that although information in the war zone is fragmentary and can be difficult to verify, reports of innocents killed “emerge within 24 hours — with graphic images of reported victims often widely disseminated across media and social media.”

The U.S.-led coalition’s policy “of downplaying or denying all claims of non-combatant fatalities makes little sense, and risks handing Islamic State and other forces a powerful propaganda tool,” Woods writes.

The report notes that the Islamic State and the regime of Syrian President Bashar Al-Assad have killed many thousands more civilians than the coalition.

The report is also a reminder of the extent to which the United States’ air war in Syria and Iraq has rolled ahead with little public debate over its effectiveness. Congress has still not passed a specific legal authorization for the war. Lawmakers punted on President Obama’s proposal this winter, which would have limited the campaign to three years and placed some limitations on sending in ground troops, and other measures introduced since then have also failed.

Much of the controversy about U.S. involvement has instead focused on training “moderate” rebel groups to fight the Islamic State in Syria. Fewer than 60 rebels have completed a Pentagon-funded training and actually gone to fight, according to the Wall Street Journal. Parallel efforts by the CIA — aimed at so-called moderate groups fighting Assad — have trained thousands of fighters over the past few years, but with little measurable success.

Yesterday, the Wall Street Journal reported that the White House has authorized airstrikes in support of Pentagon-trained rebels in northern Syria, an expansion of the current air war that could put the United States in a position of bombing Assad’s forces. So far, the coalition campaign has avoided direct confrontation with Assad.

Caption: A plume of smoke rises after an airstrike by the U.S.-led coalition against Islamic State group positions in an eastern neighborhood of Ramadi, Iraq, May 9, 2015.