UN war crimes investigators have denounced a "staggering loss of civilian life" caused by the US-backed campaign to reclaim Raqqa, the de facto capital of Islamic State.

The independent commission of inquiry tasked with investigating violations of international law, war crimes and crimes against humanity in Syria said the intensification of airstrikes by the US-led coalition had led to the deaths of at least 300 civilians in the city.

The Raqqa operation began last week with a ground assault by the Syrian Democratic Forces (SDF), an umbrella group comprising Kurdish and Arab militiamen armed by the US and supported by coalition airstrikes.

They have already pushed into Raqqa from the east and west, reportedly approaching the old city walls. Citizens have reported intense combat in areas of the city.