BEIRUT — On the outskirts of Al Omar oilfield in eastern Syria, with warplanes flying overhead, a line of trucks stretches for 6km. Some drivers wait for a month to fill up with crude.

Falafel stalls and tea shops have sprung up to cater to the drivers, such is the demand for oil. Traders sometimes leave their trucks unguarded for weeks, waiting for their turn.

This is the land of Islamic State (ISIS), the jihadi organisation in control of swaths of Syrian and Iraqi territory. The trade in oil has been declared a prime target by the international military coalition fighting the group. And yet it goes on.

Oil is the black gold that funds ISIS’ black flag — it fuels its war machine, provides electricity and gives the fanatical jihadis critical leverage against their neighbours.

But more than a year after US President Barack Obama launched an international coalition to fight ISIS, the bustling trade at Al Omar and at least eight other fields has come to symbolise the dilemma the campaign faces: How to bring down the “caliphate” without destabilising the life of the estimated 10 million civilians in areas under ISIS control, and punishing the West’s allies?

The resilience of ISIS, and the weakness of the US-led campaign, have given Russia a pretext to launch its own, bold intervention in Syria.

Despite all these efforts, dozens of interviews with Syrian traders and oil engineers, as well as Western intelligence officials and oil experts, reveal a sprawling operation almost akin to a state oil company that has grown in size and expertise despite international attempts to destroy it.

Minutely managed, ISIS’ oil company actively recruits skilled workers, from engineers to trainers and managers.

Estimates by local traders and engineers put crude production in ISIS territory at about 34,000-40,000 barrels per day. The oil is sold at the wellhead for between US$20 (S$27.6) and US$45 a barrel, earning the militants about US$1.5 million a day.

“It’s a situation that makes you laugh and cry,” said one Syrian rebel commander in Aleppo, who buys diesel from ISIS areas even as his forces fight the group on the front lines. “But we have no other choice, and we are a poor man’s revolution. Is anyone else offering to give us fuel?”

ISIS’ oil strategy has been long in the making. Since the group emerged on the scene in Syria in 2013, long before they reached Mosul in Iraq, the jihadis saw oil as a crutch for their vision for an Islamic state. The group’s shura council identified it as fundamental for the survival of the insurgency and, more importantly, to finance their ambition to create a caliphate.

Most of the oil ISIS controls is in Syria’s oil-rich east, where it created a foothold in 2013, shortly after withdrawing from the north-west — an area of strategic importance but with no oil. These bridgeheads were then used to consolidate control over the whole of eastern Syria after the fall of Mosul last year.

When it pushed through northern Iraq and took over Mosul, ISIS also seized the Ajil and Allas fields in north-eastern Iraq’s Kirkuk province. The very day of its takeover, locals say, militants secured the fields and engineers were sent in to begin operations and ship the oil to market.

“They were ready, they had people there in charge of the financial side, they had technicians that adjusted the filling and storage process,” said a local sheikh from the town of Hawija, near Kirkuk. “They brought hundreds of trucks in from Kirkuk and Mosul and they started to extract the oil and export it.” An average of 150 trucks, he added, were filled daily, with each containing about US$10,000 worth of oil. ISIS lost the fields to the Iraqi army in April but made an estimated US$450 million from them in the 10 months it controlled the area.

While Al Qaeda, the global terrorist network, depended on donations from wealthy foreign sponsors, ISIS has derived its financial strength from its status as monopoly producer of an essential commodity consumed in vast quantities throughout the area it controls. Even without being able to export, it can thrive because it has a huge captive market in Syria and Iraq.

Indeed, diesel and petrol produced in ISIS areas are not only consumed in territory the group controls but in areas that are technically at war with it, such as Syria’s rebel-held north: the region is dependent on the jihadis’ fuel for its survival. Hospitals, shops, tractors and machinery used to pull victims out of rubble run on generators that are powered by ISIS oil.

“At any moment, the diesel can be cut. No diesel — ISIS knows our life is completely dead,” says one oil trader who comes from rebel-held Aleppo who spoke to the Financial Times by telephone. FINANCIAL TIMES