Beirut (AFP) - Barrels of oil, sacks of sugar, crates piled with fruit: goods worth millions of dollars criss-cross Syria's battlefronts daily, waved through by bitter enemies who have become business partners.

Syria's regime, rebels, Kurds, and even jihadists are linking up with well-connected businessmen to turn a profit at crossings connecting otherwise divided territory.

Multiple sources from rebel-held parts of Syria including military commanders, businessmen, fighters, and residents have described to AFP a sprawling, quasi-official network of deals and arrangements on cross-country trade.

Critics say they have allowed armed groups and businessmen, some linked with President Bashar al-Assad's regime, to profit from the divisions tearing Syria apart.

- Sweet deal -

One key junction where business takes place is Morek, between the northwestern province of Idlib -- which is held by various rebel and jihadist forces -- and government-controlled Hama.

On the rebel side, Morek is managed by Al-Qaeda's onetime Syria branch Hayat Tahrir al-Sham (HTS), with the other side run by government forces.

Year round, vegetables, biscuits and clothes leave Idlib, while fuel, sugar, and spare car parts are trucked in through Hama from across swathes of government-held territory, sources at the crossing and others familiar with operations there said.

"Morek is the most important crossing between rebels and the regime, given the trade coming and going through there," said Abu al-Huda al-Sorani, who administers the border for HTS.

"It's an official transit point recognised by us both, and it's the money that makes things move."

In an interview with an AFP correspondent at the crossing, Sorani said Morek "was opened with the mediation of businessmen who have links with the regime."

"One man monopolises the trade on the regime side," he said.

Sorani declined to provide a name for the businessman, but multiple sources familiar with operations at the crossing pointed to a mysterious businessman known only as Ghawar.

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One source in opposition-held territory with close knowledge of the crossing said Ghawar pays Syrian government forces at least $1 million (850,000 euros) every few months for exclusive use of a stretch of the M5 highway leading to Morek.

The source, and others interviewed by AFP, spoke on condition of anonymity fearing a backlash from rebel or regime forces for revealing details of trade operations.

Ghawar, who acts as a frontman for regime-linked businessmen, also sets duties paid by each truck passing through loyalist checkpoints before they reach the crossing, the source said.

On the other side, HTS monopolises sugar sales in opposition zones and bans female livestock from leaving Idlib to maximise breeding in rebel areas and keep the regime dependent on them.

"No one can trade in sugar unless they're covered by HTS, because of its high revenue," the source said.

HTS also sets export fees. On July 8, an AFP correspondent saw stonemasons protesting near Morek after duties per truckload jumped from $400 to $1,500. Ultimately the demonstrators managed to force prices back down.

- Coming and going -

"With zones across Syria controlled by various forces, border trade between them has become a fait accompli," said Ayman al-Dassouky, an analyst at the Turkey-based Omran Centre.

"It brings mutual benefit to the warring sides who have allied themselves with businessmen taking advantage of the current situation to boost trade," Dassouky said.

The crossings were "generating millions for the forces which hold them and businessmen who trade across them," he said.

They provide rebels with a vital source of revenue, he said, "especially with the dwindling external support to them, mainly from the Gulf."

But trade across the front line is also crucial for the regime.

"Its forces and loyalist militias make a profit which ultimately guarantees their loyalty, and big traders close to the regime benefit from deals on duties," said Dassouky.

The phenomenon of enemies doing business together is widespread across conflicts, said Bassam Abou Abdallah, who heads the Damascus Centre for Strategic Studies.

"In all wars, not just in Syria, these guys become the warlords. A web of interests is spun between the warring sides because of economic benefits," Abou Abdallah said.

Business at Morek is so good that rival Islamists wanted a slice, and have tried in recent months to set up their own crossing from Idlib into Hama.

Ahrar al-Sham, a hardline faction once allied to HTS but which has fought against it since last year, attempted to establish trade through the Qalaat al-Madiq crossing it controls, about 30 kilometres (20 miles) west of Morek.

HTS objected and so far only a limited number of goods are passing through the Madiq crossing.

"HTS forbade large trucks from reaching the Madiq crossing so Morek would remain number one for trade," said a rebel commander based near Madiq.

- Wealth from war -

Despite being bitter enemies, Kurds and Turkish-backed rebels are also running crossings linking territory under their control.

The Hamran junction in Aleppo province is held by Kurdish militiamen on one side and the Levant Front, rebels loyal to Ankara, on the other.

Up to 60 crude oil tankers transit through Hamran from Kurdish areas daily to be refined in opposition zones, a rebel official at the checkpoint said.

Trade moves in the other direction too, with Ankara ultimately dictating what goes from rebel areas to Kurdish territory, he told AFP.

"Fertiliser is banned because it can be used to make explosives, and cement and metal too because they're used for blast walls against us," the official said.

Crossings are cash cows for well-placed businessmen, especially those bringing goods to besieged areas.

The Eastern Ghouta suburb of Damascus was surrounded for a half-decade by regime troops, with residents inside dying from lack of medicine or proper nutrition.

Government forces recaptured it in April with an assault that killed more than 1,700 civilians. But during the siege, one man controlled what trickled in.

Dairy mogul Muhyeddin al-Manfoush enjoyed exclusive access to Ghouta's market, said rebels and local businessmen.

Manfoush's "informal monopoly" over Ghouta began in 2014, said Aron Lund, a Century Foundation researcher who has written extensively about Ghouta.

"Working with both rebel and regime commanders, he quickly emerged as a pivotal figure in the area's political economy," Lund wrote in Foreign Policy last year.

The key to Manfoush's grip on Ghouta's market was the Wafideen crossing, which officially was only for regime-approved humanitarian aid.

- 'Bill Gates' of siege -

Only Manfoush was allowed to deliver barley, rice, and sugar to Ghouta, paying loyalist checkpoints up to 2,000 Syrian pounds ($4) per kilogram to let products in, multiple sources said.

"Anything he brought in was forbidden for other traders," said one businessman who paid Manfoush to bring non-food items into Ghouta.

Rebels also dealt with Manfoush, charging their own duties, the sources said.

Yasser Dalwan, an official from the Jaish al-Islam opposition faction which once controlled much of Ghouta, said rebels had no choice but to deal with the businessman.

"The regime permitted goods to enter through Manfoush. He was the designated trader," Dalwan said.

The vast wealth Manfoush was rumoured to have acquired from trade earned him the nickname "the Bill Gates of Ghouta" from residents and fighters.

And many, like rebel Abu Haytham who left Ghouta for rebel territory further north as government forces took over, are still bitter months later.

"He got rich off the siege and people's hunger," Abu Haytham said.