U.S. forces on the ground in northern Syria are helping a major offensive against the Islamic State group in its stronghold of Raqqa province, Kurdish-Arab fighters battling the jihadists say, according to AFP.

Near the frontline north of the IS bastion of Raqqa city, American commandos climbed onto a low rooftop carrying U.S.-made anti-tank missiles.

"These are U.S. special operations forces and this is why you cannot follow them or take many pictures," said a fighter with the Syrian Democratic Forces, which announced on Tuesday, May 24 an assault on the jihadists north of Raqa.

Leaning on a partially destroyed home in the village of Fatisah recently seized from IS, SDF field commander Hawkar Kobane told AFP that "U.S. forces are taking part in this operation" alongside his own troops.

"On the rooftop of this house, there are US forces using (anti-tank) TOW missiles to fire on the explosives-rigged cars that Daesh is using to attack the SDF," he said, using an Arabic acronym for IS.

The SDF has also received air support from the U.S.-led air coalition bombing the jihadists in Iraq and Syria, AP says.

And this week, the first of 250 members of the U.S. special operations forces were set to arrive in northeast Syria to support the fight, joining dozens of advisers already on the ground.

Though the Pentagon insists the commandos are working away from the frontlines, the troops' presence in Fatisah shows just how close to IS forces they are.

Pentagon press secretary Peter Cook downplayed the significance of the foreign military patches.

"Special operations forces, when they operate in certain areas, do what they can to, if you will, blend in with the community to enhance their own protection, their own security," Cook said.