More than 450 civilians have been killed in the U.S.-led airstrikes against the Islamic State group in Iraq and Syria, according to a new report from a group of independent journalists.

The group Airwars said in its report that 57 specific airstrikes resulted in the death of at least 459 civilians, including more than 100 children. The U.S. has only acknowledged two non-combatant deaths, according to the Guardian.

The strikes were identified through reporting from "two or more generally credible sources, often with biographical, photographic or video evidence" and also corresponded to confirmed coalition strikes conducted in the area at that time, Airwars said.

"Almost all claims of noncombatant deaths from alleged coalition strikes emerge within 24 hours - with graphic images of reported victims often widely disseminated," the report said, according to The Associated Press. "In this context, the present coalition policy of downplaying or denying all claims of noncombatant fatalities makes little sense, and risks handing (the) Islamic State (group) and other forces a powerful propaganda tool."

Airwars found that the largest death toll came on Dec. 28, 2014, when an airstrike hit an Islamic State headquarters doubling as a prison in Al Bab, Syria, killing 58 non-combatants. Among the dead were four women and a number of teenagers, some thought to have been jailed for buying cigarettes.

In all, Airwars said the U.S.-led coalition has conducted more than 5,800 airstrikes against Islamic State militants since the U.S. launched the assault in Iraq on Aug. 8 and in Syria on Sept. 23, according to AP.

Other groups, such as the Britain-based Syrian Observatory for Human Rights, have also identified significant civilian casualties suspected of being caused by the U.S.-led campaign. The group said in its most recent report that 173 civilians have been killed in Syria alone since airstrikes began, including 53 children.

The U.S. military outfit leading the campaign, U.S. Central Command, has to date only published one official investigation on civilian deaths. The report was published in May and found that two children were killed in a November 2014 airstrike in Syria. Since then, U.S. Central Command has conducted further investigations into three additional strikes but reported that claims of civilian deaths were "unfounded," according to the Guardian.

The coalition's lead air commander, Air Force Lt. Gen. John Hesterman, has called the campaign "the most precise and disciplined in the history of aerial warfare."

Airwars project leader Chris Woods disagreed, telling the Guardian, "The emphasis on precision in our view hasn't been borne out by facts on the ground."