A US-backed alliance has ousted Islamic State group jihadists from half of their Syrian bastion Raqa, a monitor said Wednesday, as the escalating fighting drove up the civilian death toll. 

The Syrian Democratic Forces have been fighting for several months to capture the northern city, which has become infamous as the Syrian heart of IS's so-called "caliphate."

The SDF "are now in control of 50 percent of Raqa city despite the fierce resistance mounted by IS", Rami Abdel Rahman, head of the Syrian Observatory for Human Rights, told AFP. 

The SDF assault has been backed by air strikes, special forces advisers, equipment and weapons from the US-led coalition fighting IS in Syria and neighbouring Iraq.

On Wednesday, a barrage of US-led air strikes across the city left at least 29 civilians dead, the British-based Observatory said.

"At least eight children are among the dead," Abdel Rahman said. 

The SDF's Arab and Kurdish fighters first entered Raqa on June 6, seven months after launching their flagship offensive for the city -- called Operation Wrath of the Euphrates. 

They have since steadily advanced in a pincer-like motion, closing in on the city centre.

But IS has fought back using a slew of car bombs, suicide attacks, weaponised drones and with improvised explosive devices scattered across the city.

- Tightening noose -

The spokeswoman for the SDF's Raqa operation told AFP that just under half the city was under SDF control.

"Forty-five percent of Raqa has been liberated. Our forces are advancing on all sides," Jihan Sheikh Ahmed said late Wednesday. 

"The tighter the noose grows around Daesh (IS), the more strongly it reacts and the tougher it fights," Ahmed said. 

IS first seized Raqa in early 2014, and the city has since become synonymous with the group's most gruesome atrocities.

The jihadists carried out public beheadings there and is also thought to have used Raqa as a hub for planning attacks overseas.

Tens of thousands of civilians have fled the escalating violence in recent months, but the United Nations estimates that up to 50,000 people remain trapped inside the city.

Those who have managed to escape have told harrowing tales of dodging sniper fire and mines or paying smugglers to lead them out. 

Once IS is ousted from Raqa, a body called the Raqa Civil Council is expected to run the city's administrative affairs. 

But much of Raqa's infrastructure has been devastated by years under jihadist rule and bombing by various parties in Syria, including the coalition.

- Rising civilian toll -

With Wednesday's deadly raids, at least 325 civilians, including 51 children, have died in the city since the SDF penetrated Raqa less than two months ago, according to the Observatory. 

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

A deputy commander of the international coalition said Sunday it would have "a great deal more" to do in Syria even after Raqa is captured. 

"Daesh is not defeated with the liberation of Raqa. The defeat of Daesh was not completed with the liberation of Mosul" in Iraq, British Major General Rupert Jones told reporters, using an Arab acronym for Islamic State.

More than 330,000 people have lost their lives in Syria since the country's conflict broke out in March 2011 with anti-government protests.

The widespread popular demonstrations have since evolved into a complex war drawing in world powers on all sides. 

On Wednesday, at least 14 civilians were killed when unidentified warplanes bombed territory east of the IS-held town of Mayadeen, the Britain-based Observatory said.

Five children were among the dead.

Source: AFP