US military advisors operating inside IS Syria stronghold of Raqqa

United States military advisers are operating inside the city of Raqqa, the Islamic State group's last major bastion in Syria, a US official said on 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 Raqqa were "not hundreds" and that they had been working closely with SDF fighters since operations to encircle Raqqa began.

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

With help from the US-led coalition, the SDF this month breached an ancient wall by Raqqa'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 Raqqa offensive.