Amnesty International has urged the US-led coalition battling ISILin Syria to conduct a thorough investigation into civilian deaths in the 2017 campaign to liberate the Syrian city of Raqqa.

The coalition’s admission last month that it killed 77 more civilians than previously reported was just the "tip of the iceberg”, the rights group said in a statement published on Tuesday.

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

"Every family I met in Raqqa has one question which was ‘why did the coalition bomb us?’,” said Amnesty’s adviser Donatella Rovera in the statement.

"It is time that the coalition stops being in denial and does the right thing: a proper independent investigation.”

Amnesty also said it believed hundreds of civilians were killed and that the coalition had under-reported the casualties.

There was no immediate comment from the Global Coalition, a group formed by 77 countries whose mission is to degrade and ultimately defeat the Islamic State of Iraq and the Levant (ISIL, also known as ISIS).

The coalition needs to release "meaningful and verifiable information” about how targets in Raqqa were selected and how strikes were carried out, the statement said.

"How can the coalition avoid inflicting high civilian death tolls in the future without accounting for what went wrong?” said Rovera in the statement.

Aljazeera