The US-led coalition fighting the Islamic State (IS) militants in Iraq and Syria on Friday acknowledged another 21 civilian deaths in past air strikes, bringing the toll up to 624.

In its monthly findings, the coalition said it had completed investigations into 132 allegations of civilian casualties.

"To date, based on information available, (the coalition) assesses that, it is more likely than not, at least 624 civilians have been unintentionally killed by coalition strikes" since the anti-IS campaign began in late 2014, the coalition said Friday.

Aside from probing civilian death reports that come in from coalition pilots and through social media and other channels, military investigators also continued wading through a huge backlog of hundreds of allegations reported by the website Airwars.org.

The London-based collective of journalists and researchers has always had civilian death tolls that are wildly divergent from those acknowledged by the coalition.

According to the most recent Airwars tally, 4,734 civilians have been killed in coalition strikes.

Among the incidents the coalition said resulted in casualties was a May 12 strike near Raqa that targeted IS fighters but also killed 10 civilians in a nearby building.

"Although all feasible precautions were taken and the decision to strike complied with the law of armed conflict, unintended civilian casualties unfortunately occurred," the statement read.

As of the end of June, the military was still assessing 424 civilian casualty reports.