RAQA - 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 Airstrikes 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 Airstrikes 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 Airstrikes.

"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.

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.

EU sanctions 16 more Syrians over chemical attacks

The European Union on Monday imposed sanctions against 16 more high-ranking military Syrian officials and scientists over chemical weapons attacks on civilians, a statement said.

The move by the bloc’s foreign ministers brings to 255 people now facing a travel ban and an assets freeze over President Bashar al-Assad’s violent crackdown on civilians during a five-year civil war. “The EU added these 16 persons for their role in the development and use of chemical weapons against the civilian population,” an EU statement said.

The EU will release the names of those hit by the sanctions on Tuesday, it said.

The UN’s chemical watchdog, the OPCW, last month concluded that sarin was used as a chemical weapon in the April 4 attack in the Syrian town of Khan Sheikhun that killed at least 87 people including children.

The sanctions decision “shows the resolve of the UK and the rest of our friends in Europe in dealing with those who are responsible for chemical weapons attacks,” British Foreign Secretary Boris Johnson told reporters just before the decision was announced.

Syria is already subject to an oil embargo, restrictions on certain investments, a freeze of the assets of the Syrian central bank held in the EU, as well as export restrictions.

It also is under sanctoins on equipment and technology that might be used for internal repression as well as on equipment and technology for the monitoring or interception of internet or telephone communications.