The UN said Monday that it is looking into reports that as many as 40 civilians were killed in an airstrike near Ramadi last Friday, an incident that could be the latest deadly attack to hit innocent bystanders in the campaign by the Iraqi government and a US-led coalition against the self-styled Islamic State (IS).

The UN's statement came on the same day that the watchdog group Airwars said in a report that the estimated civilian death toll from the coalition's air campaign in both Iraq and Syria now stands at at least 459 and possibly as high as 1,086. A narrower category, which included only incidents that coincided with reported coalition airstrikes and for which "fair" public reporting was available, put the higher number at 591. Those figures were for incidents from August of last year until the end of June. The US-led coalition has thus far admitted culpability for only two civilian deaths in its air campaign.

In a statement, the UN's Assistance Mission for Iraq (UNAMI) said the special representative for Iraq, Jan Kubis, "expressed serious concern at reports that up to 40 civilians may have been killed and over 30 wounded in an airstrike in Rutba, west of Ramadi, on July 31."

"According to preliminary reports, three houses hosting internally displaced persons were hit in the alleged airstrike," said UNAMI. "Children were reportedly among the casualties." The mission asked the Iraqi government to investigate the incident.

The US-led coalition did not report launching airstrikes in the vicinity of Rutba on July 31, but the report of civilian casualties was also not immediately refuted — something the coalition has done in the past when VICE News inquired about similar allegations.

A spokesperson for the coalition said four airstrikes did take place "near" Ramadi, which is about 200 miles east of Rutba in Anbar province, an area that is predominantly controlled by IS. According to Airwars, which maintains a running account of airstrikes, local reports indicated the "source of the attack was disputed," with some outlets suggesting an Iraqi government plane carried out the strike. A spokesperson for the coalition told VICE News that it was "not aware of Iraqi Government airstrikes in that area [Rutba] on July 31."

One of the incidents mentioned in the Airwars report was an attack that allegedly occurred on April 30 in Ber Mahli, a town in Syria's Aleppo province. According to the watchdog group's investigators, 64 civilians were killed in that strike. In early May, a coalition spokesperson told VICE News that airstrikes did take place in the vicinity of Ber Mahli on April 30, but they said the bombs destroyed IS fighting positions and hit 50 militants.

'This is not the first time we have received unconfirmed reports about civilians being killed by operations.'

Using an alternate acronym for IS, the coalition spokesperson added that, "Prior to the airstrikes, Kurdish forces, who held the town before leaving after being attacked by ISIL, reported there were no civilians present in that location and that there had not been any civilians present for the two weeks prior to coalition airstrikes." A large death toll in the town was subsequently confirmed to VICE News by several human rights groups, who said the only likely culprit was the coalition. If the coalition was in fact responsible for the civilian deaths, it was unclear if the intelligence from Kurdish forces was faulty or purposely erroneous.

UNAMI spokesperson Eliana Nabaa told VICE News the mission is still investigating Monday's attack. "We still don't have confirmation about who these airplanes belonged to," she said.

"This is not the first time we have received unconfirmed reports about civilians being killed by operations," she added.

Airstrikes in Iraq and Syria have only increased in recent months; in July there were at least 371 strikes reported by the coalition, the highest one-month toll since the bombing began last year.

But some of the coalition's airstrikes appear to go unreported. One of the deadliest attacks included in the Airwars report was a December 28 strike on an IS building that was being used as a temporary prison in Al Bab, a town in Aleppo.

Citing numerous media and NGO reports, Airwars counted at least 58 non-combatant deaths that resulted from the strike. Among the dead were prisoners who had been locked up by IS for buying cigarettes and other petty crimes. After the attack, activists claimed it was widely known that IS was using the building as a prison, and not for operational activities. It took the coalition until January 10 to admit to carrying out the December 28 strikes in Al Bab, an admission that came after an investigative report by the US newspaper group McClatchy.

"On other occasions the Coalition has failed to identify strikes on a particular town or city, only for individual allies to then report such an attack," Airwars wrote. The report cited one reporting period — on October 18 and 19— when the French military reported an airstrike in Tikrit, while the British did the same for an attack in the vicinity of Ramadi. Neither was included in US Central Command airstrike summaries.

"In regard to civilian casualty allegations, there is no other military in the world that works as hard as we do to be precise," a coalition spokesperson told VICE News by email. "When an allegation of civilian casualties caused by Coalition forces is determined to be credible, we investigate it fully."

The American military has carried out four investigations into alleged civilian casualties, only one of which led to an admission that two deaths had resulted from airstrikes. Two other investigations by US Central Command are still underway.

According to Airwars, some 53 incidents that allegedly killed at least 233 civilians "warrant urgent investigation by the Coalition."

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