Fighters from the Syrian Democratic Forces (SDF) walk along a road dotted with black veils ditched by women after they crossed over from Islamic State group territory near Tishreen Farms, on the northern outskirts of Raqa on May 2, 2017

Dozens of black veils dotted a freshly laid sand berm in northern Syria, ditched by women fleeing the Islamic State group's bastion of Raqa as US-backed fighters close in.

Outside the village of Tishreen Farms, 17 kilometres (10 miles) north of Raqa, the Syrian Democratic Forces could be seen laying sandbags to protect themselves from IS car bombs and snipers.

With air support from the US-led coalition, the alliance of Kurdish and Arab fighters has seized swathes of territory from the jihadists, who were stationed less than a kilometre away.

SDF fighters told AFP that women hastily shed their IS-mandated black veils after crossing into SDF territory near Tishreen Farms, revealing vibrant, patterned robes underneath.

"Most of the women tear off their robes and burqas as soon as they arrive at our positions," an SDF fighter said, speaking on condition of anonymity.

"Some of the women stomp on the robes because they finally feel safe and are finished with Daesh," he added, using the Arabic acronym for IS.

Thousands of civilians have been smuggled out of Raqa and surrounding territory in recent weeks.

Ahmad, who fled Raqa to a village near Tishreen Farms, told AFP that he was one of the lucky ones.

He managed to escape despite IS's brutal measures to block residents from leaving.

"IS is using the civilians as human shields to protect itself," said the man in his 30s.

"We fled as part of a group two days ago, and an IS sniper shot and killed two of us."

- Key coalition raids -

In Tishreen Farms, male and female SDF members worked to fill an underground tunnel they say jihadists used to avoid coalition air strikes as they transported supplies and ammunition throughout the area.

IS has used that tactic -- as well as weaponised drones and car bombs -- to defend territory across the so-called "caliphate" it declared in 2014.

The SDF launched its fight for Raqa in November, just a month after US-backed forces in neighbouring Iraq announced an offensive for IS's other main stronghold, Mosul.

Both assaults have received crucial support from the US-led air coalition bombing the jihadists.

The bodies of alleged IS fighters were still visible around Tishreen Farms, along with destroyed vehicles on the side of the road that testified to the heavy strikes that targeted the area.

One SDF fighter told AFP that coalition raids had killed "most of the IS fighters present in these villages".

"Others were killed during our combing operations... IS has lost a huge part of its defensive capabilities. The fight isn't as intense," he said.

As it faces mounting pressure, IS has lashed out in other areas -- including multiple suicide attacks near a refugee camp on the Syrian-Iraqi border on Tuesday that left at least 46 people dead.

- 'For its people' -

The SDF has already seized most of Raqa province as part of its "Wrath of the Euphrates" campaign, named after the major river that cuts across the northern part of Syria.

At their closest point, they are just eight kilometres (five miles) from Raqa city.

SDF spokesman Talal Sello said SDF fighters were still working on fully besieging the city, after which they would launch the final phase of the campaign.

"As our troops get closer and closer to Raqa city, the number of soldiers and advisers from the international coalition continues to increase," Sello told AFP.

The US has dispatched about 900 troops to Syria to help train and advise the SDF, as well as a Marine artillery unit.

Ahmad al-Hassan, a clean-shaven local SDF commander, said the US-led coalition "has provided the SDF with special weapons including artillery, tanks, anti-tank missiles."

Like many SDF fighters, he wore a scarf around his head to protect from the reddish dust in the air from an incoming sandstorm.

The SDF would fully liberate the city in coordination with the coalition, Hassan said, but "Raqa will only be for its people."