Updated

A leading international watchdog on Tuesday urged the U.S.-led coalition battling the Islamic State group in Syria to further investigate civilian deaths in the 2017 campaign to liberate the Syrian city of Raqqa.

According to a statement by Amnesty International, the coalition's admission last month that it killed 78 more civilians than previously reported is just the "tip of the iceberg."

An investigation is needed to understand why civilians were killed and who was responsible, the watchdog said, adding that Raqqa residents deserved justice and compensation.

Amnesty also said it believed hundreds of civilians were killed and that the coalition had underreported the casualties. There was no immediate comment from the coalition.

The statement quoted Amnesty's adviser Donatella Rovera as saying the coalition needs to release "meaningful and verifiable information" about how targets in Raqqa were selected and how strikes were carried out.

"How can the coalition avoid inflicting high civilian death tolls in the future without accounting for what went wrong in Raqqa," she said.

The coalition had initially put the civilian death toll at 32 in the campaign in Raqqa, the de-facto capital of the Islamic State group's so-called "caliphate."

But an Amnesty investigation published in June and based on testimony collected from Raqqa residents prompted the coalition last month to add 78 civilians to the death toll, 49 of whom Amnesty said were women and children.

Amnesty has also criticized the coalition's monthly reports, which offer only generalities about the strikes.

"Such short-hand explanations are woefully inadequate," it said.

The battle for Raqqa, once a city of 200,000 people, played out over four months in 2017, with the coalition playing a supporting role as the Kurdish-led Syrian forces fought street by street. The coalition unleashed wave after wave of airstrikes and shell fire until the last of the militants left Raqqa in October 2017.

The city was left in ruins, bodies scattered in the streets. Civil workers pulled nearly 500 corpses from the rubble in the immediate aftermath and were still finding more months after the fighting.

IS has been mostly defeated in Iraq and Syria, though it still controls small pockets of territory in both countries.