Working together in the open air, Abu Ahmad's family uses buckets to empty what appears from a distance to be a pit of crude oil outside the rebel-held Syrian town of Al-Bab.

The source of the winter fuel these farmers will use is not crude, however, but olives.

Abu Ahmad and his clan sun-dry olive paste mixed with water which will supply their stoves as a substitute for heating oil, now unaffordably expensive due to the country's raging conflict.

"Our ancestors used to do the same," the 80-year-old says, smiling toothlessly from beneath his keffiyeh, the traditional Arab headdress, and peering out from eyes magnified by his thick glasses.

"When the fighting started, fuel got too expensive. So last year we began drying out the paste again.

"I remember when I was a kid this was the only fuel available to us. It's called jift."

Their home in the northeast of Aleppo province lies near to Al-Bab, which is targeted daily by regime air raids.

Women family members, veiled and wearing gloves, happily wade to waist height into the strong-smelling black sludge that fills a hole dug in the ground between rows of olive and pistachio trees.

The women form a chain to pass full buckets of the gooey stuff along to load onto a tractor, helped by giggling children covered from head to toe in the paste.

The menfolk look on as they chain smoke.

They agree to tell journalists about the process, but will not allow them to photograph or film the women.

One of Abu Ahmad's sons, 37-year-old Mustafa, explains how the olives are pressed then pumped through a hose into a large hole. The paste is covered in water, left during the winter and extracted for drying at the end of summer.

The moist olive paste is spread out on the ground in big square patterns that resemble a freshly painted black oil on canvas.

After a few days in the late summer sun, the paste dries up, cracks and separates into smaller parts which are gathered into large plastic bags.

"In the stove this burns more effectively than wood or anything else, because of the oil content," Mustafa says.

"We burn it on its own or with logs. It releases a lot of heat, and better still it smells good."

The price of heating fuel, which was once subsidised, has increased almost tenfold from a pre-conflict price of seven pounds (10 cents) per litre to 65 pounds, or just less than a dollar as summer ends and Syrians prepare for another cold winter.

"Heating oil is cleaner and more practical. You don't need to go to all this effort to obtain it. But we can't afford it anymore," Abu Ahmad says.

"Jift is not sold, it is up to us to make it. We share with with all the members of the family and sometimes give some to the neighbours as well, to those who aren't able to keep warm in winter," he says.

"It can get to five degrees below zero here in January. We're close to the Turkish mountains," adds Mustafa.

"If we don't work hard in the summer, we'll freeze in the winter."