U.S.-led airstrikes targeting the Islamic State group in Iraq and Syria have likely killed at least 459 civilians over the past year, a report by an independent monitoring group said Monday. The coalition had no immediate comment.

The report by Airwars, a project aimed at tracking the international airstrikes targeting the extremists, said it believed 57 specific strikes killed civilians and caused 48 suspected “friendly fire” deaths. It said the strikes have killed more than 15,000 Islamic State militants.

While Airwars noted the difficulty of verifying information in territory held by the IS group, which has kidnapped and killed journalists and activists, other groups have reported similar casualties from the U.S.-led airstrikes.

“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 U.S. launched airstrikes in Iraq on Aug. 8 and in Syria on Sept. 23 to target the Islamic State group. A coalition of countries later joined to help allied ground forces combat the extremists. To date, the coalition has launched more than 5,800 airstrikes in both countries.

The U.S. has only acknowledged killing two civilians in its strikes: two children who were likely slain during an American airstrike targeting al-Qaida-linked militants in Syria last year. That same strike also wounded two adults, according to an investigation released in May by the U.S. military.

That strike is the subject of one of at least four ongoing U.S. military investigations into allegations of civilian casualties resulting from the airstrikes. Another probe into an airstrike in Syria and two investigations into airstrikes in Iraq are still pending.

Airwars said it identified the 57 strikes through reporting from “two or more generally credible sources, often with biographical, photographic or video evidence.” The incidents also corresponded to confirmed coalition strikes conducted in the area at that time, it said.

The group is staffed by journalists and describes itself as a “collaborative, not-for-profit transparency project.” It does not offer policy prescriptions.

“The coalition’s war against ISIL has inevitably caused civilian casualties, certainly far more than the two deaths Centcom presently admits to,” the group 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.”

In Iraq, the U.S.-led coalition includes France, Britain, Belgium, Netherlands, Australia, Denmark and Canada. Jordan has also carried out airstrikes in Iraq as well as in Syria, although it has released no further information about the dates or locations of its attacks.

The coalition conducting airstrikes in Syria include the U.S., Saudi Arabia, Jordan, Bahrain and the United Arab Emirates. Canada began its own strikes in April, while Britain carries out routine reconnaissance-only drone missions above Syria, and British pilots have carried out airstrikes while embedded with U.S. forces.

Airwars called for greater transparency and accountability from coalition members, since each is individually liable 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.

Other groups also have reported on major casualties suspected of being caused by the U.S.-led airstrikes. The Britain-based Syrian Observatory for Human Rights, which relies on a network of on-the-ground activists, said 173 Syrian civilians have been killed since airstrikes began, including 53 children under the age of 18. Most of the civilians were killed in airstrikes near oil refineries and oil fields in the northern provinces of Hassakeh, Raqqa, Aleppo and Deir el-Zour.

The Observatory said the deadliest incident was on May 4, when a U.S.-led airstrike on the northern Islamic State-controlled village of Bir Mahli killed 64 people, including 31 children. A Pentagon spokesman at the time said there was no information to indicate there were civilians in the village. The death toll was confirmed by other opposition groups in Syria.

Two videos and several photos released by a media arm of the IS group purport to show the aftermath of the strikes in the mixed Arab and Kurdish village showed children allegedly wounded in the airstrikes.

In another incident on June 8, an airstrike likely conducted by the U.S.-led coalition on the Islamic State-held village of Dali Hassan, also in northern Syria, killed a family of seven, the Observatory said.

Turkey, which recently began carrying out its own airstrikes against the IS group in Syria and Kurdish militants in northern Iraq, said it would investigate accusations by the Iraqi Kurdish regional government and activists with the Kurdistan Workers’ Party, or PKK, that its airstrikes caused civilian casualties in the northern Iraqi town of Zargel.

The United Nations said Monday that it is concerned about reports that 40 civilians may have been killed and over 30 wounded in an airstrike west of Ramadi in Iraq’s Anbar province, and called on the Iraqi government to investigate the incident.

Also on Monday, the leader of Iraq’s Kurdish region, President Massoud Barzani, said Iraqi Kurds must maintain control of areas in northwestern Iraq, including the city of Sinjar, after they are recaptured from IS militants.

His speech marked the anniversary of the fall of Sinjar to the Islamic State group, which forced tens of thousands of people from Iraq’s Yazidi religious minority to flee into the mountains, prompting the U.S. to begin the airstrikes targeting the militant group.

Other Kurdish groups, including the PKK and the Syrian Kurdish People’s Protection Units, claim Sinjar as part of their territory. All three groups are battling to retake Sinjar.

(AP)