Airstrikes by the U.S.-led coalition appear to have killed hundreds of civilians in Syria, a leading rights group said on Tuesday, urging the Pentagon to share more information about alleged civilian casualty incidents.

Following a detailed study, Amnesty International concluded that a series of attacks from September 2014 to July 2016 killed at least 300 people. Some of the raids by the United States and its allies, the group said, may have violated international humanitarian law.

“Our understanding is that there have been far greater civilian deaths than have been accounted for,” said Neil Sammonds, a Syria researcher with Amnesty. “For the most part it doesn’t look as if they’ve done adequate, thorough investigations, so we would like to be assured that they would be done.”

Amnesty provided the findings to the Pentagon in a memo in September but has received no response, the group said. U.S. Central Command (CENTCOM), which oversees operations in Syria and is responsible for examining civilian casualty allegations, said it was examining the allegations linked to the 11 purported incidents, which Amnesty selected because they were seen to be particularly clear-cut instances of civilian deaths.

“The coalition takes great care — from analysis of available intelligence to selection of the appropriate weapon to meet mission requirements — in order to minimize the risk of harm to noncombatants,” Maj. Josh Jacques, a Centcom spokesman, said in an email.

The findings from Amnesty come as the United States intensifies its air campaign against Islamic State targets, especially around the Iraqi city of Mosul, amid what U.S. officials hope will be a decisive victory against the militant group. But more than two years after air operations began against the group, the Obama administration is forced to contend with a growing roster of reported civilian casualties.

While human rights groups say that Russian and Syrian air attacks are responsible for the vast majority of deaths in Syria, Centcom has received around 250 allegations of civilian casualties in Iraq and Syria since 2014. Officials have completed 62 investigations, concluding that 55 civilians were killed across the two countries.

“Amnesty International is concerned that CENTCOM appears to be significantly underestimating the extent to which the operations it directs have caused harm to civilians and damage to civilian objects,” the group said in its memo.

Amnesty said it conducted its own investigations into the 11 incidents, sifting through social media and activist reports and, when possible, interviewing eyewitnesses or relatives of the alleged victims. “We think this was pretty robust under the circumstances,” Sammonds said.

The group has also asked for details about what information Centcom has used and has disregarded when looking into allegations. Officials sometimes use local reporting and social media images and video when conducting official investigations, but sometimes rely mainly on military and intelligence information.

Amnesty is also urging the Obama administration to overhaul its investigation process, which has not provided full details to the public. Several completed investigations have been made public in a redacted form, but most have not beyond a summary of their conclusions.

Jacques said the command is committed to transparency. “We are working to release the assessment findings of the remaining closed allegations as soon as possible,” he said.