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Civilian casualties in coalition operations against the Islamic State group in Iraq and Syria are being significantly underreported, monitoring groups say, with non-US partners refusing to release data about military operations in which their forces may have killed non-combatants.

Since the anti-IS international coalition was formed in August 2014, there have been at least 21,820 airstrikes in Iraq and Syria, according to monitoring group Airwars.

The US first reported civilian casualties resulting from airstrikes in May 2015, and has in total admitted responsibility for 377 civilian deaths.

Most recently, a Pentagon investigation revealed that a single airstrike on Mosul in March killed at least 105 civilians, although officials blamed the high death toll on a secondary explosion of Islamic State group munitions.

Since October 2014, however, the US Central Command (Centcom) has left it up to individual countries to identify their own airstrikes, Airwars says, with most coalition partners failing to release specific data while also claiming to have avoided civilian casualties.

On April 30, however, the US Central Command released its' latest report on civilian casualties from coalition operations. While setting out in relative detail civilian deaths resulting from US airstrikes, at least 80 civilian deaths were attributed to coalition forces, but without mentioning specific nations.

Centcom officials told Airwars that partner nations were responsible for the 80 deaths, but the US was pressured against releasing more detailed information about airstrikes by ally nations, Foreign Policy reported.

Since the anti-IS international coalition was formed in August 2014, there have been at least 21,820 airstrikes in Iraq and Syria, according to monitoring group Airwars

"In reference to the 80, those do reference non-US strikes," one official told Foreign Policy. No information was released about when or where the deaths occurred, or how many civilians died in each incident.

In a further blow to transparency, new US protocol will no longer include details of its own airstrikes which result in civilian casualties, instead listing only 'coalition' operations as a way to shield allies from identification, Foreign Policy reported.

"We will just say 'Coalition,' and we won't say if it was US or not," Centcom Director of Public Affairs Col. John Thomas said.

Airwars contacted all 12 members of the US-led coalition as part of its investigation into responsibility for the 80 civilian deaths detailed in the US Centcom report.

Australia, Britain, Canada, and Denmark all denied involvement while Bahrain, Jordan, Saudi Arabia, Turkey and the UAE did not respond, Foreign Policy reported.

After the United States, Britain is by far the most active member of the coalition, carrying out more than 1,200 airstrikes in Iraq and Syria. The British government has so far boasted of "zero civilian casualties" in what is termed the "most precise" air campaign in history.

Airwars says it has raised 120 incidents with the British Ministry of Defence where UK forces may have been involved in civilian casualties, Foreign Policy reported, but most were investigated and closed.

When pressed about responsibility for any of the 80 non-US deaths, a Ministry of Defence spokesman pointed to a March report claiming that it had "not seen evidence that we have been responsible for civilian casualties so far," Foreign Policy reported.

While the US has admitted hundreds of civilian casualties since 2014, Airwars believes the figure could be as high as 3,500.

Recent months have seen record death tolls in Syria and Iraq from coalition bombings, with April said to be the worst month for civilian casualties since operations began.