The assault on the city by the SDF adds to pressure on the jihadist group in Syria, which also faces another offensive by Russian-backed regime troops in its bastion province of Raqqa. (Photo: AP)

Halula: US-backed fighters advanced Sunday to within five kilometres (three miles) of the Islamic State group's stronghold of Manbij in northern Syria, threatening a crucial supply link for the jihadists.

The assault on the city by the Syrian Democratic Forces (SDF) adds to pressure on the jihadist group in Syria, which also faces another offensive by Russian-backed regime troops in its bastion province of Raqqa.

Supported by air strikes by the US-led coalition battling IS in Syria and Iraq, the SDF alliance of Kurdish and Arab militias has been making steady gains since launching the operation to take Manbij last week.

Read: Iran-backed militia may enter Fallujah if fight against ISIS drags on

The Syrian Observatory for Human Rights, a Britain-based monitoring group, said Sunday the SDF was "now within about five kilometres of the strategic city of Manbij".

Manbij is located along a route connecting Raqqa -- IS's de facto capital in Syria -- to the Turkish border, a vital conduit for supplies and foreign terrorists.

US Central Command spokesman Colonel Patrick Ryder said Saturday SDF fighters had seized more than 100 square kilometres (40 square miles) of territory during the advance.

More than 55 air strikes have been carried out since the offensive began, he said, adding that the goal was to hamper IS's ability "to move fighters, weapons, finances (and) supplies into and out of Syria and Iraq".

Some 3,000 Arab fighters were taking part, backed by around 500 Kurdish militia members, he said, adding that US special forces were working "at the command and control level" in the operation.

In the village of Halula 27 kilometres (16 miles) east of Manbij, an AFP correspondent saw several US soldiers in jeeps as they assisted SDF fighters. An AFP photographer last week took several pictures of US special forces in the area.

Washington has said US forces are advising the SDF on the ground but not taking direct part in combat.

In Halula, in an area of dry hills dotted with occasional trees, the AFP reporter saw dozens of civilians who had fled areas around Manbij, including many children, most with few belongings.

Sitting under a tree with her nine children, Jawaher said she was from a village near Manbij that had been under IS control.

"They lived near us and we had to do what they said or they would kill our children or take our homes," she said.

The United Nations says at least 20,000 civilians have fled the fighting around Manbij.

At least 74 people have died in fighting since the start of the offensive last Monday, including 32 civilians mainly killed in coalition air strikes, said the Observatory, which relies on a network of medics and activists to monitor the conflict.

Thirty jihadists were also killed, it said, along with 12 SDF fighters.

After taking the village of Khirbet al-Rus, about 20 kilometres (12 miles) southeast of Manbij, the SDF rescued a group of Yazidis -- six women and 16 children -- who were being held captive by IS, the Observatory said.

They were among hundreds of Yazidis taken in mid-2014 as IS carried out a brutal campaign of massacres, enslavement and rape against the Yazidi minority.

The timing of the offensive -- as Syrian troops advanced into IS-held territory -- has raised suspicions that Moscow and Washington are covertly coordinating operations by their respective Syrian allies.

On Saturday, forces loyal to Syrian President Bashar al-Assad pushed into IS's bastion province of Raqqa in northern Syria.

With Russian air support, Syrian troops pushed into the province from the southwest, moving to within 40 kilometres (25 miles) of the Euphrates Valley town of Tabqa, site of the country's biggest dam.

Since starting with a 2011 crackdown on anti-government protests, Syria's conflict has evolved into a complex, multi-front civil war that has left more than 280,000 dead and forced millions from their homes.

IS emerged from the chaos of the war in mid-2014, seizing control of large parts of Syria and neighbouring Iraq, declaring a fundamentalist Islamic "caliphate" and committing widespread atrocities.

Iraqi forces have also been steadily regaining ground against the jihadists, and late last month began a major offensive to retake the city of Fallujah, just 50 kilometres (30 miles) west of Baghdad.

Diplomatic efforts to get Syria's regime and non-jihadist rebels to move towards peace have been thwarted by a lack of trust and continued fighting, especially around second city Aleppo, which is divided between government and rebel control.

Dozens of fresh regime strikes on opposition-held parts of Aleppo on Sunday killed at least 23 civilians, the Observatory said.

Nine civilians were killed when a crude barrel bomb hit the Qaterji neighbourhood, where an AFP journalist saw a street strewn with rubble as residents ran for safety and a rescuer rushed a bloodied child into an ambulance.