At least 224 civilians have been killed in US-led coalition air strikes since the Syrian forces it backs entered ISIL's stronghold of Raqqa a month ago, a monitoring group said.

The Syrian Observatory for Human Rights (SOHR) said on Thursday that at least 38 children and 28 women were among the air raid victims, adding that it did not have a toll for civilians killed in other ways, including by other military operations, mines, or while trying to flee the city.

Arab and Kurdish fighters from the Syrian Democratic Forces (SDF) broke into Raqqa on June 6 after a months-long operation to encircle the Islamic State of Iraq and the Levant (ISIL, also known as ISIS) group in the northern city.

SOHR said clashes and air strikes in Raqqa had killed 311 ISIL fighters and 106 SDF fighters since June 6.

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Tens of thousands of civilians are believed to be trapped inside Raqqa, with warnings that the armed group is using them as human shields.

Many Raqqa residents who have managed to escape said ISIL snipers are targeting anyone trying to leave the city.

The US-backed coalition is providing the SDF's campaign on Raqqa with heavy air support, as well as special forces advisers, weapons, and equipment.

Since ISIL captured Raqqa in early 2014, it has served as the de facto capital of the armed group's Syrian territory.