More than 5,700 air strikes have been launched in the campaign, which will mark its first anniversary on Saturday.
According to a new report by Airwars, a nonprofit team of independent journalists aimed at tracking airstrikes, 52 attacks caused at least 459 non-combatant deaths, including those of more than 100 children.
"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.
"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."
The strikes have killed more than 15,000 IS militants, the report said.
The US launched its airstrike campaign against IS in Iraq on August 8 and in Syria on September 23. A coalition of countries later joined to help allied ground forces combat the extremists, Reuters reported.
In one of the worst cases of civilian deaths, in Al Bab, Syria, a US strike on a local IS headquarters being used as a jail killed up to 58 non-combatants, including women and teenagers, according to the report.
US Central Command, however, has said to date that the only civilian deaths were those of two children killed in a November 2014 strike in Syria. That same strike also wounded two adults, according to a US report released in May.
"The coalition's war against ISIL has inevitably caused civilian casualties, certainly far more than the two deaths Centcom presently admits to," Airwars says on its website.
"Yet it's also clear that in this same period, many more civilians have been killed by Syrian and Iraqi government forces, by Islamic State and by various rebel and militia groups operating on both sides of the border."
Airwars called for greater transparency and accountability from coalition members, each of which is individually responsible for any civilian deaths or injuries it causes.
"Only one of twelve coalition partners – Canada – has consistently stated in a timely fashion both where and when it carries out airstrikes," the report said.
