Civilians fleeing the Islamic State group's Syrian bastion of Raqa are paying smugglers to lead them out of the city safely, only to find themselves caught up in harrowing attacks.

Tens of thousands of civilians have fled Raqa and the surrounding province since the US-backed Syrian Democratic Forces (SDF) began an offensive to capture the jihadist stronghold last year.

But the journey is perilous, with IS preventing civilians from abandoning its shrinking territory and enforcing its ban with deadly violence.

In the Ain Issa camp, where thousands of people displaced from Raqa have sought refuge, many civilians say they were promised safe passage by smugglers who ended up leading them into deadly situations.

Ali, 25, asked around in his village of Qahtaniya, about six kilometres (nearly four miles) northwest of Raqa, to find a smuggler.

"The smugglers don't reveal their real names, they use false ones. Ours called himself 'Al-Hout' (The Whale)," he told AFP.

"I agreed with The Whale on the details and I paid him 222,000 Syrian pounds ($418)," he said, an amount that covered him and eight family members, the youngest of them a five-year-old child.

On the night of their escape, they left the village in Ali's car at 2:00 am and headed for a rendezvous point where the smuggler and other fleeing civilians were waiting.

The smuggler moved ahead of the group and then brought them to a halt.

"We arrived at a place that he told us was safe, and then we came under a barrage of Daesh gunfire," Ali said, using an Arabic acronym for IS.

He gunned his car engine and managed to reach a nearby SDF checkpoint safely, but a woman who had joined them at the meeting point was wounded in the attack.

"We don't know what happened to her, but the blood of those (who have died fleeing) is on the hands of the smugglers," he said.

- Captured by IS -

The Kurdish and Arab fighters of the SDF entered Raqa in June and have since captured more than 40 percent of the city from IS.

But the UN estimates that between 20,000 and 50,000 civilians remain trapped inside in dire conditions.

The UN says civilians are paying between 75,000 and 150,000 Syrian pounds ($150-$300) per person to smugglers to organise their escape.

When Ahmed al-Hussein, 35, decided to flee Raqa, he agreed to pay a smuggler 70,000 Syrian pounds and hand over his motorbike, worth another 30,000 Syrian pounds, when he reached safety.

"We were a group of 250 people. We were moving for 15 hours until we reached an SDF checkpoint in the Al-Mazeila area, 23 kilometres (15 miles) northwest of Raqa, at the break of dawn," he told AFP.

"As soon as we arrived, the jihadists launched an attack on the area, and we were caught in the crossfire. The smugglers disappeared completely."

IS captured him and other civilians and took them to the village of Al-Salihiya, where they beat them and stole their money and identity papers.

They also interrogated the civilians, trying to learn who had smuggled them out.

"It was impossible because we really had no idea. The smugglers could have been Daesh themselves," he said.

- Intercepted by smugglers -

Hussein was eventually released, and moved to another IS-held village, remaining there until it was captured by the SDF several months ago.

Throughout the ordeal he managed to hang onto his motorbike, which sat next to him in his tent.

All of those interviewed by AFP about their experiences fleeing Raqa with smugglers declined to be interviewed on camera, citing fears not only of the smugglers but also of IS.

Elsewhere in the camp, 38-year-old Abu Ahmed was repairing tyres and motorbike parts to bring in a little cash to supplement the meagre supplies available to the displaced.

He fled the Daraiya neighbourhood of Raqa three months ago, but opted to make his own way without resorting to a smuggler.

"I left Raqa with my family before the SDF arrived. We stayed on a nearby farm for about a month until Daesh fighters came and burned our tents," he said, his shirt stained with oil.

The family managed to escape and headed through the desert, searching for the nearest SDF checkpoint.

"Before we reached the checkpoint, we were intercepted by smugglers who asked us to pay them 50,000 Syrian pounds to help us, saying the road ahead was long and unsafe," Abu Ahmed said.

But as he was talking with the smugglers, a shepherd approached him.

"Don't give them your money," the shepherd told him.

"The checkpoint is no more than 500 metres (yards) from here, and the road is safe."