Observers say the Western coalition has killed 1.600 civilians in Syria.

Angus McDowall (Reuters) - A U.S.-backed airstrike to expel Islamic State from Raqqa, its capital in Syria, killed more than 1.600 civilians, 10 times more than the coalition itself admitted, Amnesty International and the monitoring group Airwars said on Thursday.

Amnesty International and Airwars, a London-based group created in 2014 to monitor the impact of the US-led campaign against the Islamic State, spent 18 months researching civilian deaths, including two months in Raqqa, they said.

"Our conclusive finding after all this is that the military offensive by the US-led coalition (American, British and French forces) directly caused more than 1.600 civilian deaths in Raqqa."

They said the cases they documented likely amount to violations of international humanitarian law and called on coalition members to create a fund to compensate the victims and their families.

In response to the report, the coalition said it is taking "all reasonable measures to minimize civilian casualties" and that there are still allegations that it is investigating.

“Any accidental loss of life during the defeat of Daesh is tragic,” said Scott Rawlinson, a spokesman for the coalition, in a statement emailed late Thursday local time, using an Arabic abbreviation for Islamic State.

“However, this needs to be balanced against the risk of allowing Daesh to continue terrorist activities, causing suffering and loss to whomever they choose,” he added.

The Islamic State seized Raqqa in early 2014 during its lightning advance through Syria and Iraq, and established a self-proclaimed caliphate characterized by the summary execution of opponents.

Additional reporting by Idrees Ali in Washington.