The Assad regime reportedly struck an ominous deal with ISIS to take back a strategic city

New documents obtained by Sky News revealed that the Syrian government's recapture of the ancient city of Palmyra from Islamic State militants was apparently part of a pre-arranged deal that allowed ISIS to remove its heavy weaponry from the city before withdrawing.

Sky News reported that the documents came from from a Free Syrian Army group comprised of ISIS defectors originally from Raqqa, ISIS' de facto capital in Syria.

"Withdraw all heavy artillery and anti-aircraft machine guns from in and around Palmyra to Raqqa province," read one document that was dated just before the Syrian Arab Army recaptured Palmyra at the end of March.

Stuart Ramsay, Sky News' chief correspondent, said he asked one of the defectors if ISIS was coordinating its movements directly with forces loyal to Assad - and even with Russia, which backed the assault on Palmyra with heavy airstrikes.

"Of course," the ISIS defector told Ramsay.

The documents obtained by Sky News provide more evidence that the Assad regime has been colluding with the jihadists, who have captured more than half of Syria's territory since 2012.

The Wall Street Journal reported last month on files uncovered during a raid on the home of Abu Sayyaf, the Islamic State "oil minister" who was killed by US Special Forces at his compound in Syria's Deir Ezzour province last May. The files revealed deals the Assad regime supposedly made with Sayyaf that, at one point, contributed up to 72% of ISIS' profits from natural resources.

Abu Sayyaf's division had successfully negotiated agreements with the Assad regime to allow Islamic State trucks and pipelines to move from regime-controlled fields through territory controlled by the group, which is also known as ISIS, ISIL, or Daesh. The division helped the jihadists bring in roughly $40 million a month in oil sales alone, according to documents seen by The Journal.

The natural-gas fields surrounding Palmyra were a particularly important source of revenue for the jihadists. They turned the gas into fuel which they then sold to Assad, according to Matthew Reed, the vice president of Foreign Reports Inc., a Washington, D.C.-based consulting firm focused on oil and politics in the Middle East.

The documents smuggled out of Raqqa to Sky News appeared to provide corroboration.

"One document requests safe passage for a driver through IS checkpoints 'until he reaches the border with the Syrian regime to exchange oil for fertilizer,'" Ramsay wrote.

He added: "The defectors claim that this is a trade agreement between the two sides that has been going on for years."