Hundreds of Islamic State fighters, including Pakistanis and Europeans, escaped from the terror group’s former stronghold of Raqqa in Syria as part of a deal brokered by Kurdish fighters and backed by US-led foreign troops, BBC has reported.

A convoy of almost 50 trucks, 13 buses and more than 100 IS vehicles left the bombed out Raqqa on October 12, “under the gaze of the US and British-led coalition and Kurdish-led forces who control the city”, the report said. Some of IS’s “most notorious members” and “dozens of foreign fighters” were part of the convoy and there are fears that the fighters could return to their own countries.

The Syrian Democratic Forces (SDF), an alliance of Kurdish and Arab fighters, told truck drivers who joined the convoy that they were to take hundreds of displaced families from the town of Tabqa to a camp further north.

But when the drivers assembled the convoy on October 12, they realised “they had been lied to” as their “deadly cargo” comprised “hundreds of IS fighters, their families and tonnes of weapons and ammunition”. The dozens of drivers who were part of the convoy were promised thousands of dollars but told the matter had to remain secret.

It was initially understood that no foreign fighter would be allowed to leave Raqqa alive. But foreign fighters were able to join the convoy, according to the drivers. One driver told the BBC: “There was a huge number of foreigners. France, Turkey, Azerbaijan, Pakistan, Yemen, Saudi, China, Tunisia, Egypt...”

The report said, “The deal to let IS fighters escape from Raqqa – de facto capital of their self-declared caliphate – had been arranged by local officials. It came after four months of fighting that left the city obliterated and almost devoid of people. It would spare lives and bring fighting to an end. The lives of the Arab, Kurdish and other fighters opposing IS would be spared.

“But it also enabled many hundreds of IS fighters to escape from the city. At the time, neither the US and British-led coalition, nor the SDF, which it backs, wanted to admit their part.”

Some of the foreign fighters have spread out across Syria, and some have sneaked across the border into Turkey, BBC reported after speaking to dozens of people “who were either on the convoy, or observed it, and to the men who negotiated the deal”.

One driver was quoted as saying: “We were scared from the moment we entered Raqqa…We were supposed to go in with the SDF, but we went alone. As soon as we entered, we saw IS fighters with their weapons and suicide belts on. They booby-trapped our trucks. If something were to go wrong in the deal, they would bomb the entire convoy. Even their children and women had suicide belts on.”

The SDF cleared Raqqa of media so that the escape of the IS fighters from their base would not be televised. Publicly, the SDF said only a few dozen fighters, all of them locals, were able to leave.

But one driver said this wasn't true: “We took out around 4,000 people including women and children - our vehicle and their vehicles combined. When we entered Raqqa, we thought there were 200 people to collect. In my vehicle alone, I took 112 people.”

Another driver said the convoy was six to seven kilometres long and IS fighters, their faces covered, sat on top of some of the vehicles. BBC also reported there was secretly filmed footage that showed lorries towing trailers crammed with armed men.

“Despite an agreement to take only personal weapons, IS fighters took everything they could carry. Ten trucks were loaded with weapons and ammunition,” the report said.

Following the BBC investigation, the US-led coalition has admitted the part it played in the deal. Some 250 IS fighters were allowed to leave Raqqa, with 3,500 of their family members.

“We didn’t want anyone to leave,” said Col Ryan Dillon, spokesman for Operation Inherent Resolve, the Western coalition against IS.

“But this goes to the heart of our strategy, ‘by, with and through’ local leaders on the ground. It comes down to Syrians – they are the ones fighting and dying, they get to make the decisions regarding operations,” he said.

One driver was quoted as saying: “They said, 'Let us know when you rebuild Raqqa - we will come back…They were defiant and didn’t care. They accused us of kicking them out of Raqqa.”

The fighters left the convoy inside IS-held territory, somewhere between Markada and Al-Souwar. Since then, some of the foreign fighters have been taken across the Turkish-Syrian border by smugglers. One smuggler told BBC he had overseen the smuggling of 20 families in one week alone. The families included French, Europeans, Chechens and Uzbeks.

Abu Musab Huthaifa, the IS intelligence chief who was in the convoy and was captured near the Turkish border, provided details of the negotiations on October 10 that allowed the fighters to leave Raqqa.

“Air strikes put pressure on us for almost 10 hours. They killed about 500 or 600 people, fighters and families,” he said.

“After 10 hours, negotiations kicked off again (on October 11). Those who initially rejected the truce changed their minds. And thus we left Raqqa,” he added.

Huthaifa said the convoy went to the countryside of eastern Syria, not far from the border with Iraq, and thousands escaped.