At least 801 civilians have been killed by United States (US)-led coalition airstrikes against the militant Islamic State (IS) group in Iraq and Syria, according to a Pentagon report released Thursday.

Five more strikes, all in Syria, were investigated over the past month and found to have resulted in 15 additional civilian deaths, the report stated.

However, monitoring groups say the number of civilian casualties from coalition airstrikes in the fight against IS is far higher than the Pentagon acknowledges.

Since August 2014, more than 5,000 civilians have been killed as a result of coalition airstrikes in Iraq and Syria, according to the London-based Airwars organisation that tracks civilian deaths in the anti-IS operation.

The majority of civilian casualty allegations are currently from coalition airstrikes in Syria.

The Pentagon says they are still assessing 695 reports of civilian casualties, more than 400 of which are from strikes carried out in Syria.

US-backed Syrian forces retook the city of Raqqa from IS control in October and pushed IS fighters out of a swath of territory along the Euphrates river valley in the following weeks.

Each investigation that found an allegation credible determined it was “more likely than not” that a coalition strike resulted in a civilian casualty, the report stated.

“Although all feasible precautions were taken and the decision to strike complied with the law of armed conflict, unintended civilian casualties unfortunately occurred,” the Pentagon added.

US-led coalition airstrikes are credited with fuelling advances against IS by Iraqi and Syrian ground forces.

Since the coalition's campaign of airstrikes against IS was launched in August 2014, IS-held territory has been reduced to pockets of desert along the Iraqi-Syrian border.