The Washington-led alliance puts the toll at just over 1,000 civilians in both Syria and neighbouring Iraq, and says it does all it can to prevent civilian deaths.

The coalition began bombing IS targets in Iraq in August 2014 after the jihadist group seized swathes of territory straddling the two countries, proclaiming an Islamic "caliphate".

The coalition extended its strikes to Syria on September 23, 2014.

The Syrian Observatory for Human Rights, a Britain-based monitoring group, said on Sunday those Syria strikes had since killed 3,331 civilians.

The monitor relies on a network of sources inside Syria and tracks flight patterns, aircraft involved and ammunition used to determine who carries out raids.

"Among those killed are 826 children and 615 women," said Observatory chief Rami Abdel Rahman.

The coalition says it takes every possible precaution to prevent civilian deaths.

In its latest civilian casualty report published last month, the coalition said its strikes had unintentionally killed 1,061 civilians in both Iraq and Syria up until July 30, 2018.

It is still assessing a further 216 reports of civilian casualties, some of them in strikes dating back to 2014.

Its spokesman did not immediately respond to an AFP request for comment on the discrepancy between its toll and that of the Observatory, or for a breakdown of civilian casualties in Syria.

The coalition's operations have largely wound down, with the jihadists ousted from all but tiny bits of territory in Syria.

More than 360,000 people have been killed across Syria since the conflict broke out in 2011, nearly a third of them civilians, according to the Observatory.