Washington (AFP) - United States military advisers are operating inside the city of Raqa, the Islamic State group's last major bastion in Syria, a US official said Wednesday.

The troops, many of them special operations forces, are working in an "advise, assist and accompany" role to support local fighters from the Syrian Democratic Forces as they battle IS, said Colonel Ryan Dillon, a military spokesman.

The troops are not in a direct combat role but are calling in air strikes and are working closer to the fight than did US forces supporting the Iraqi military in Mosul.

"They are much more exposed to enemy contact than those in Iraq," Dillon said.

He said the numbers of US forces in Raqa were "not hundreds" and that they had been working closely with SDF fighters since operations to encircle Raqa began.

After months of capturing villages and surrounding terrain, operations to liberate Raqa started in November and on June 6 the SDF entered the city.

With help from the US-led coalition, the SDF this month breeched an ancient wall by Raqa's Old City, where diehard jihadists are making a last stand.

The SDF are a Kurdish-Arab alliance that is being trained by coalition experts and armed by the United States.

Dillon said the coalition had seen IS increasingly using commercial drones that have been rigged with explosives. The jihadists employed a similar tactic in Mosul.

"Over the course of the last week or two, it has increased as we've continued to push in closer inside of Raqa city center," he said.

The US military is secretive about exactly how big its footprint is in Syria, but has previously said about 500 special operations fighters are there to train and assist the SDF.

Additionally, Marines are operating an artillery battery to help in the Raqa offensive.

- Civilian deaths -

Meanwhile, an independent monitoring group said the civilian death rate soared last month as operations intensified in Raqa and Mosul.

Airwars -- a London-based collective of journalists and researchers that uses social media, eyewitness reports and other sources to compile its data -- said the concurrent assaults on the cities were often "devastating."

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The group said as many as 744 civilians had been killed last month, though the number is completely at odds with that of the US military, which so far has acknowledged the accidental deaths of 603 civilians since coalition operations began in late 2014.

Airwars director Chris Woods said the increased tempo of strikes in Mosul and Raqa accounted for some of the increase, but suggested the Pentagon's goal of "annihilation" of the jihadists had placed civilians at greater risk of harm -- a claim the military disputes.

Dillon said a seven-person team reviews every civilian casualty allegation, many of which are self-reported through the military itself.

The process "is very thorough and I will take our credible reports over any of the allegations that do come in," he said.