DAMASCUS • Hundreds of Islamic State in Iraq and Syria (ISIS) fighters and their family members were allowed to leave the terror group's de facto capital of Raqqa in Syria, as an alliance of Kurdish and Arab fighters closed in last month, under a secret deal, according to a BBC expose.

The deal was brokered by local officials after four months of fighting left the city devastated and almost devoid of people.

The logic was to spare lives and bring the fighting to an abrupt end, safeguarding Syrian Democratic Forces (SDF) fighters opposed to ISIS. But the arrangement also enabled hardened ISIS militants to escape from the city and disperse far and wide across Syria, and possibly overseas.

United States Defence Secretary James Mattis had described the fight against ISIS as a war of "annihilation" in May. And after Raqqa was liberated, coalition forces said that only a few dozen fighters had been able to leave, all of them locals.

But one of the lorry drivers who spirited the militants out of Raqqa revealed: "We took out around 4,000 people, including women and children - our vehicles and their vehicles combined. When we entered Raqqa, we thought there were 200 people to collect. In my vehicle alone, I took 112 people."

The convoy carrying the group was 6km to 7km long, according to another driver, with almost 50 trucks, 13 buses and more than 100 of ISIS' own vehicles.

Ten trucks were loaded with weapons and ammunition alone.

And belying the claims that foreign fighters were not allowed to leave, a lorry driver told the BBC: "There was a huge number of foreigners. France, Turkey, Azerbaijan, Pakistan, Yemen, Saudi, China, Tunisia, Egypt..."

One lorry driver said he and other truckers were subjected to three days of hard driving. Some of the ISIS fighters and even their family members had suicide belts strapped around their waists as they sat perched in the back of the trucks. Some of the drivers were even tortured by the militants they were transporting. "We were scared from the moment we entered Raqqa," said the driver. "We were supposed to go in with the SDF, but we went alone. As soon as we entered, we saw ISIS 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 US-led coalition now admits that the deal took place. "We didn't want anyone to leave," said Colonel Ryan Dillon, spokesman for Operation Inherent Resolve, the Western coalition against ISIS. "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."

While it was ISIS' capital, Raqqa was also a cage for the hardened militants. Now that the militants are free, there exists a distinct possibility that many of them are not done fighting.