The group, Airwars, which describes itself as a "not-for-profit transparency project" said in a report posted on its website on Monday that it had evidence to suggest that between 459 and 591 civilians had been killed in the US-led air campaign since it was launched one year ago.

The report, entitled "Cause for Concern," said that these had come in 57 incidents, in which it believed there was enough evidence on the public record to indicate coalition responsibility for their deaths. It added that 48-80 members of "friendly forces" had also been killed.

The report noted that the fact that the United States has so far only conceded that two non-combatants had likely been killed in the air campaign "indicates a worrying lack of urgency on the part of all Coalition members regarding civilian deaths."

"Almost all claims of non-combatant deaths from alleged Coalition strikes emerge within 24 hours –with graphic images of reported victims often widely disseminated across media and social media. In this context, the present Coalition 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," the report concluded.

A year of airstrikes

Almost a year ago, the United States began conducting airstrikes against "Islamic State" (IS) jihadists in Syria, who have seized control of large swathes of territory in the country. In September 2014, the US expanded its campaign to target IS positions in northern Iraq. Later, 12 US allies, including Britain and France but also Saudi Arabia, Jordan and the United Arab Emirates joined the coalition, which has launched more than 5,800 airstrikes in Syria and Iraq in an effort to support ground forces fighting against the militants.

Airwars said its primary source for the data used in the report was military briefings from the US and its allies taking part in the airstrikes. It also used information reported by other monitoring groups, such as the London-based Syrian Observatory for Human Rights.

pfd/jil (AP, dpa)