Heavy bombardment and fierce fighting shook the Islamic State group's Syrian stronghold Raqa on Monday, as US-backed forces said they captured a new neighbourhood from entrenched jihadists.

Bursts of gunfire and artillery as well as the thud of air strikes conducted by the US-led coalition filled the air in western neighbourhoods of Raqa, on what AFP's correspondent said was the heaviest day of bombardment to date.

The Syrian Observatory for Human Rights, updating an earlier toll, said the air strikes killed at least 10 civilians, two of them children.

Thick pillars of black smoke dotted the city skyline, lined with bombed-out concrete homes and the damaged minaret of a mosque.

"Our American friends are bombing with mortars," a fighter from the Syrian Democratic Forces (SDF) said after a string of blasts near the front line in western Raqa.

The SDF's Kurdish and Arab fighters have been pressing an operation to capture the jihadist stronghold since last year with coalition air support and the backing of a US Marines artillery battery.

An AFP reporter in Jazra suburb on the western outskirts of the city on Monday saw coalition forces at a joint position with SDF fighters firing artillery in the direction of IS posts deeper inside Raqa.

- Regime forces advance south of Raqa -

On another front, Syrian government forces have swept across territory south of Raqa, capturing a string of villages and oilfields from the jihadists, state media said Monday.

"Over the past two days, army units recaptured a number of villages and oilfields in western parts of Deir Ezzor province and southern parts of Raqa," the official news agency SANA said.

The Syrian Observatory said the government advance was backed by heavy Syrian and Russian air strikes.

"Regime forces have captured between 1,500 and 1,800 square kilometres (580 and 700 sq miles) in Raqa province over the past 48 hours," Observatory head Rami Abdel Rahman said.

In its separate operation, the SDF has spent several months encircling Raqa and first broke into the northern city in early June.

On Monday, it announced it had captured Al-Yarmuk, a large neighbourhood on the southwestern outskirts of the city.

"The Al-Yarmuk district was liberated yesterday," the SDF's spokeswoman for the Raqa operation, Jihan Sheikh Ahmed, told AFP in Ain Issa, 50 kilometres (30 miles) north of Raqa.

"We are taking steady and sound steps. What is important to us is not speed, but liberating civilians and eliminating Daesh (IS)," she added.

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

It carried out brutal public beheadings and is thought to have used Raqa as a hub for planning attacks overseas.

The jihadists are putting up a fierce defence against the SDF, including with car bombs, improved explosive devices, and weaponised commercial drones.

SDF fighters on Monday heard a light buzzing noise coming from outside and began looking to the sky.

"It's a Daesh drone," one militiaman warned.

Progress inside Raqa has also been hampered by extensive mining of neighbourhoods, with devastating consequences for civilians trying to flee.

"There have been many casualties, fighters and civilians, caused by mines," an SDF commander told AFP, without giving his name.

"Yesterday, we buried six civilians after a mine exploded as they were trying to escape."

- Up to 50,000 civilians still trapped -

The United Nations estimates that up to 50,000 civilians remain trapped in Raqa, down from some 100,000 people at the end of June.

The SDF on its social media accounts said Monday its forces "managed to free about 500 civilians who were trapped inside the Al-Daraiya and Al-Tayar neighbourhoods, as well as 150 others from the Old City" in Raqa.

The Syrian Observatory monitoring group said hundreds of civilians had fled IS-held parts of the city in the past 48 hours towards areas now controlled by the SDF.

"Whenever there is a lull in the fighting, they leave towards areas held by the SDF," Observatory head Abdel Rahman said.

The Britain-based monitoring group estimates the US-backed force currently holds around 35 percent of Raqa.

It said the SDF held the western portion of Al-Yarmuk district but that it had not fully captured the district and heavy fighting was continuing.

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

source: AFP