The Islamic State group had struck a deal with the Syrian regime to withdraw its fighters from the ancient city of Palmyra, leaked documents cited by Sky News revealed Monday. In March, Syrian forces retook Palmyra, which was held by the extremist group, also known as ISIS, for nearly a year.

The documents leaked by ISIS defectors revealed that Syrian President Bashar Assad and ISIS had colluded with each other. There were arrangements made for ISIS fighters to evacuate some areas before the Syrian army attacked. Also, the alleged collusion included a deal to trade oil for fertilizer.

"Withdraw all heavy artillery and anti-aircraft machine guns from in and around Palmyra to Raqqa province," one of the documents, which are copies of handwritten orders sent from ISIS headquarters, reportedly read.

Palmyra, a Unesco world heritage site located northeast of the Syrian capital city, Damascus, was taken back by the Russian-backed Syrian army from ISIS fighters last month.

Afzal Ashraf from the think tank, Royal United Services Institute, told Sky News: "It may take 20 years before what we know exactly what is going on.

"Almost certainly there will be some sort of communication going on between mortal enemies, and that is for short-term tactical gains and losses," Ashraf said. "Certainly if there is economic trading going on, which we know there is, there would be communication."

The documents also reportedly suggest that the program of recruiting and training ISIS fighters to carry out attacks against the West began long before the security services of these nations had anticipated.