As Islamic State group fighters steadily lose chunks of their Syrian bastion Raqa to a US-backed force, the jihadists are ramping up the ferocity of their counter-attacks.

The Syrian Democratic Forces (SDF) first broke into Raqa in early June and have advanced in a pincer-like motion towards the heart of the city.

The alliance's Arab and Kurdish fighters now hold half of Raqa, but as they tighten the noose around IS, the jihadist group appears to be lashing out.

"The closer we get to the city centre, the harder IS defends itself, because it's completely besieged," said Davram Dersem, an SDF field commander.

To defend Raqa, IS has deployed a barrage of car bombs, suicide bombers, weaponised drones, snipers, and mines scattered across the city.

"They're cornered like a wounded animal. Raqa is their main stronghold -- they're not going to abandon it easily," Dersem added.

The Kurdish commander spoke to AFP in the western Raqa neighbourhood of al-Daraiya.

Mortar shells crashed into surrounding neighbourhoods, which were also hit by the occasional air strike.

After IS captured Raqa in 2014, the group transformed the city into a symbol of its most macabre practices, including public beheadings.

Raqa was also thought to have been used as a hub for planning attacks overseas.

Now, much of it has been destroyed by the fierce fighting and US-led air strikes. Roofs have collapsed and streets are littered with rubble, metal, and glass.

- 'Life-or-death battle' -

In the adjacent district of Massaken al-Dubbat, 24-year-old SDF fighter Talal Sharif pointed at a devastated row of two-storey homes ahead.

"All of this destruction, it's because of their car bombs. There have been at least four in each of these streets," Sharif told AFP.

"Little by little, they're being suffocated in Raqa. This is why they're resisting."

Sharif spoke confidently, but his face was marked by exhaustion after weeks of street-by-street battles.

When his unit recently stormed an IS-held neighbourhood, they stumbled on four enemy fighters sleeping inside a home.

"During the raid, one of the jihadists blew himself up, another two were killed, and one was taken prisoner," Sharif recalled.

But if they don't have access to belts of explosives or car bombs, IS fighters resort to something much simpler -- grenades.

"In close combat, they just toss grenades. For them, it's a life-or-death battle," Devrem said.

Up to 50,000 civilians remain trapped in Raqa in increasingly dire circumstances, with little access to food, water, or life-saving medication, according to the United Nations.

The intensifying fight for Raqa has also forced tens of thousands of its residents to flee, dodging IS sniper fire, mines, and even US-led coalition air strikes.

On Friday, the Syrian Observatory for Human Rights said at least 21 civilians -- including eight children from a single family -- had been killed in "intensifying air strikes by the coalition" over the previous 24 hours.

The Britain-based monitor says more than 300 civilians including dozens of children have died since the SDF first broke into Raqa.

Another 467 IS jihadists and 219 SDF have also been killed in the fighting.

SDF advisor Nasser Hajj Mansour said the battle for Raqa is far from over.

"It could still be long. In the coming days, the battles will become more ferocious," Mansour said.

"IS jihadists will either try to hide amongst the civilians or fight until the end."

Source: AFP