Just after sunrise outside Syria's Raqa, dozens of Islamic State group jihadists donned Kurdish military uniforms, piled into cars with weapons, and broke back into a district they lost control of months ago.

The raid was the latest evidence that IS, despite holding just a pocket of territory in its one-time stronghold, retains the capacity to wage deadly guerrilla attacks on areas thought to be relatively safe.

The jihadists opened fire on a media centre run by the US-backed Syrian Democratic Forces in Al-Meshleb, the easternmost neighbourhood of Raqa and the first area taken from IS in June.

"They were wearing YPG (Kurdish People's Protection Units) uniforms and came in six cars from the east," said the centre's head Kani Shahani, who was communicating with AFP during the attack.

The YPG forms the backbone of the SDF, a key force fighting IS in northern Syria with support from the US-led coalition against the jihadists.

Hours of clashes left at least two SDF members dead and appeared to shake the confidence of the alliance of Arab and Kurdish fighters in the area as a safe rearbase.

The road into the east -- which had seen regular civilian traffic over the past few weeks -- has remained sealed for days, with large metal barrels and sand berms blocking access.

Tense SDF fighters were on high alert as they searched cars and motorcycles at checkpoints on Friday.

Two other failed infiltration attempts also hit the city's west this week. 

On Friday, jihadists were suspected of trying to sneak out of the city among civilians, SDF members said.

More than a dozen vans of SDF fighters could be seen speeding from a western suburb into the city to reinforce regular positions. 

- Sleeper cells, tunnels -

"There has been more than one attempt from Daesh to break this siege on those inside, including striking behind the front lines," said SDF press chief Mustefa Bali, using the Arabic acronym for IS.  

He said the attack on Al-Meshleb was waged by at least 30 IS fighters who snuck in from territory a few kilometres east of the city limits. 

"This shows that the infiltration was major. This area is supposed to be relatively secure," Bali said, adding that it was proof of "sleeper cells" in the area.

The SDF is tightening the noose on IS in a final pocket of territory that include Raqa's national hospital and the football stadium. 

While most of the city's neighbourhoods are under SDF control, the US-backed fighters said that they have not cleared them house by house. 

Some feared IS was using a massive network of underground tunnels to move around in secret. 

Jeker Derek, an SDF fighter in the city's west, said that his unit had fought off a surprise attack by IS earlier this week.

"Two days ago, we faced an attack from underground. A group of Daesh fighters came out from here and we clashed with them," the lean fighter said. 

He was pointing to a now-sealed tunnel entrance in a building just hundreds of metres (yards) west of Raqa's national hospital, where hundreds of jihadists are thought to be holed up with civilians. 

"This is the entrance, but it's not clear where it ends."

- 'Come from anywhere' -

Other SDF members and representatives of the US-led coalition backing the offensive on Raqa said IS was trying to blend in with displaced civilians to escape the city undetected before waging new attacks.

The coalition said last week it had detained several IS fighters -- including a local leader -- attempting to flee Raqa alongside residents. 

"Trying to sneak in with IDP (internally displaced people) flows, sneak out and lay low is something we're worried about," a senior commander from the US-led coalition backing the SDF's operations told AFP.

The commander said recent guerrilla attacks proved that IS was still a threat, but that it was lashing out in a last-ditch attempt to do damage to the SDF. 

"It's that boxer on the ropes throwing punches -- he's still going to get one in every once in a while. It's still dangerous," the commander said.

In Hawi al-Hawa, a western suburb of Raqa under SDF control for months, fighter Jamal Mahmoud seemed nervous that his unit would be the next target. 

He had heard about the hit-and-run attack on Al-Meshleb earlier in the week and said it showed that "an attack could happen at any moment".

"On the frontline, you know your enemy is going to be facing you," he said.

"But in a liberated city that has all these tunnels, he could come at you from anywhere."