Rights group says more must be done to prevent fatalities and calls for independent investigations into possible violations of international law

The US-led coalition fighting Islamic State in Syria has killed at least 300 civilians in airstrikes, and must do more to prevent further civilian deaths as the battle for Mosul intensifies in neighbouring Iraq, Amnesty International has warned.

More than half of the victims identified in a new report by the rights group, which looked at deaths over the two years since western forces joined the fight against Isis in Syria, were killed during the fight for the city of Manbij.

It is important the lessons from those deaths are learned as a broad coalition of troops backed by US air power and special forces push towards Mosul, a much bigger city that is still home to about 600,000 civilians, the group says.

“In its backing of anti-Isis ground forces during this summer’s Manbij campaign, the US-led coalition killed some 250 or more civilians, and yet it does not acknowledge them,” said Neil Sammonds, Amnesty’s researcher for Syria.

“We fear that since it is not recognising or learning from such mistakes that in the campaign for Mosul, which is far greater than Manbij, the coalition is on course to kill even greater numbers of civilians and risk a serious backlash.”

Facebook Twitter Pinterest Syria Democratic Forces fighters greet civilians evacuated from an Isis-controlled neighbourhood of Manbij. Photograph: Reuters

The rights group spent months carrying out witness interviews, collecting photographic and video evidence, analysing satellite images and reviewing reports from local activists to determine the total number of deaths.

It is a conservative toll compared with estimates from other monitoring groups, which put the number of deaths from coalition bombing at 600 to 1,000. The monitoring groups include the Syrian Network for Human Rights, the Syrian Observatory for Human Rights and the Violations Documentation Centre.

Only a handful of deaths have been acknowledged by the coalition in its attacks in Syria. Amnesty is calling for more precautions before attacks, and independent investigations into possible violations of international law.

“We fear the US-led coalition is significantly underestimating the harm caused to civilians in its operations in Syria,” said Lynn Maalouf, deputy director for research at Amnesty International’s Beirut office.

Facebook Twitter Pinterest Smoke plumes rise after a series of airstrikes in Mosul. Photograph: Feher/Zuma/Rex/Shutterstock

“Some of these attacks may constitute disproportionate or otherwise indiscriminate attacks … It’s high time the US authorities came clean about the full extent of the civilian damage caused by coalition attacks in Syria.”

Three attacks near Manbij this summer were particularly deadly, killing more than 100 civilians, at least a third of them children. One of the attacks is being investigated by US authorities.

The scale of deaths is lower than from bombing raids by Russia, the other foreign power to have sent its jets to Syria. Russian raids have killed at least 3,600 civilians in just over a year and appear to have targeted civilians, while the coalition has been trying to avoid or limit civilian casualties, although it could do far more, human rights groups said.