"The regime is paying Jabhat al-Nusra to protect oil and gas pipelines under al-Nusra's control in the north and east of the country, and is also allowing the transport of oil to regime-held areas," an intelligence source said.
The source accepted that the relationship was opportunistic, but added that the deals confirmed that "despite Assad's finger-pointing" his regime was to blame for the rise of al-Qaeda in Syria.
Meanwhile, top prosecutors and forensic experts claim to have direct evidence of "systematic torture and killing" by Syrian forces. A report released yesterday includes thousands of photographs of the dead bodies of 11,000 detainees allegedly killed in Government custody, many of whom were severely malnourished and showed signs of strangulation.
The group of prosecutors, who have have vast experience in the prosecution of war crimes and crimes against humanity, claim this is "compelling" evidence they say could be brought before an international tribunal.
"This is a smoking gun," said David Crane, the first chief prosecutor of the Special Court for Sierra Leone and one of the report's authors. "This is direct evidence of the regime's killing machine."
The 31-page report, which was commissioned by Carter-Ruck solicitors in London acting for Qatar, is being made available to the UN, governments and human rights groups.