US forces on the ground in northern Syria are helping a major offensive against the Islamic State group in its stronghold of Raqa province, Kurdish-Arab fighters battling the jihadists say.

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

"These are US 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 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 "US 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 US-led air coalition bombing the jihadists in Iraq and Syria.

And this week, the first of 250 members of the US 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.

Several US commandos were photographed wearing the military insignia of the Kurdish People's Protection Units (YPG), which make up the bulk of the SDF.

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.

Observers questioned the wisdom of such a display of support for Kurdish fighters whom neighboring Turkey -- a NATO ally and vital US partner in the region -- considers an offshoot of the Kurdistan Workers' Party (PKK).

"On a human level, I get it. They are probably doing it in some way to try and present (themselves) as a friendly ally to the locals," said Charles Lister, a senior fellow at the Middle East Institute.

"But the broader reality here is that US-Turkish relations are already on a pretty low level, and I know for a fact that something like this will have stirred significant anger in Ankara -- and that's not a good thing."

Emile Hokayem, a Middle East analyst at the International Institute for Strategic Studies, wrote on Twitter that the move was "politically tone-deaf and counterproductive in this context."

- 'A lot of experience' -

An AFP photographer saw as many as 20 US soldiers in Fatisah on Wednesday and heard them communicating in English.

They refused to speak to journalists but generally appeared less wary than usual about the media presence.

The US soldiers could be seen climbing onto pickup trucks fitted with heavy machine guns and driving across the swathes of agricultural plains that make up northern Raqa province.

Other US soldiers surveyed territory alongside SDF forces identified by the patches on the arms of their uniforms.

"The American forces present here have a lot of experience," Kobane said.

"We will take advantage of their experience to fight terrorism and capture the other villages as quickly as possible with as few casualties as possible."

Another SDF field commander, Baraa al-Ghanem, said US fighters were "present at all positions along the front... They are taking part on the ground and in the air."

"We have a joint operations room with the coalition. We also have special weapons, both heavy and light, and we are facing the problem of mines," he added.

The US forces on Wednesday could be seen accompanying a special unit within the SDF known as the "counterterrorism forces."

The two forces entered a building on the edge of Fatisah used as the town's school, reportedly to carry out a training session on using US weaponry.

Kurds play a dominant role in the US-backed SDF, providing the core of the forces that have pushed back IS in the country's northeast.

The SDF has a total of about 25,000 Kurdish fighters and around 5,000 Arab fighters.

Washington is pushing to bring more Arab forces into the group.

The Syrian war erupted in early 2011 after Bashar al-Assad's forces launched a brutal crackdown on anti-government protests, and has since claimed more than 270,000 lives.