U.S.-led airstrikes targeting ISIS Iraq and Syria have likely killed at least 459 civilians over the past year, a report by an independent monitoring group said Monday. 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 ISIS militants.While Airwars noted the difficulty of verifying information in territory held by ISIS, which has kidnapped and killed journalists and activists, other groups have reported similar casualties from the U.S.-led airstrikes. The U.S. launched airstrikes in Iraq on Aug. 8 and in Syria on Sept. 23 to target the ISIS. 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.U.S. Army Col. Wayne Marotto, a spokesman for the coalition, did not address the report directly, but said "there is no other military in the world that works as hard as we do to be precise." "When an allegation of civilian casualties caused by Coalition forces is determined to be credible, we investigate it fully and strive to learn from it so as to avoid recurrence," he said in a statement emailed to the Associated Press.Navy Capt. Jeff Davis, a Pentagon spokesman, said the department has seen the press reports on the additional civilian casualties but said the Pentagon will have nothing to say until the reports are reviewed. U.S. Central Command has finished four investigations into alleged civilian casualties, concluding that three were unfounded and that two innocent civilians were killed and two other people wounded in the fourth case.There are six other investigations still ongoing. 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 ISIS 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 ISIS and by various rebel and militia groups operating on both sides of the border."