Syrian President Assad has colluded with ISIS to arrange deals on the battlefield, according to papers seen by Sky News.
Leaked files suggest that one example of co-operation was an agreement that the jihadist group would withdraw from the ancient city of Palmyra to allow Assad's regime to recapture it. The documents seemed to show the Syrian army was only able to take the city with ISIS' support.
The leaked files appeared to confirm claims from Western-backed opposition groups that Assad did deals with ISIS to defeat the rebels.
Analysis of the letters, which came in addition to 22,000 files handed over by defectors last month, also reveals the militant group has been training jihadis to attack Western targets for longer than security services previously thought.
The documents are handwritten instructions from ISIS headquarters in Raqqa, Syria, leaked by defectors. They also claim a trade arrangement between Assad's government and ISIS has been going on for years.
"This is a war of perception and narrative and everyone is trying to manipulate events," Dr Afzal Ashraf from the think tank Royal United Services Institute told Sky.
"It may take 20 years before we know exactly what is going on," he said.
"Almost certainly there will be some sort of communication going on between mortal enemies, and that is for short term tactical gains and losses.
"Certainly if there is economic trading going on, which we know there is, there would be communication."
According to Sky analysis, ISIS militants need to maintain control of territory that they label a caliphate in order to train foreign fighters.
