Isis has reportedly been working together with Bashar al-Assad's forces in a cooperation deal to limit the Islamist militants' losses. Documents backed by interviews with Isis defectors apparently show how Assad managed to recapture Palmyra only after getting help from the jihadists who pulled out their heavy weapons and withdrew.
The claims were made by Sky News which has analysed a leak of 22,000 Isis documents also showing other agreements going back years including exchanging oil and fertiliser with the regime.
There had been existing claims from both the opposition and the west that Assad's regime did deals with Isis to weaken the opposition group. But one order said to have come before Assad retook the city was for Isis to "withdraw all heavy artillery and anti-aircraft machine guns from in and around Palmyra to Raqqa province", the news channel reported.
Afzal Ashraf from the think tank Royal United Services Institute told Sky: "This is a war of perception and narrative and everyone is trying to manipulate events.
"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."
It comes as a new study shows Isis is targeting civilians in its suicide bombings as it tries to strengthen its position in Iraq and Syria.
Meanwhile the US Secretary of State John Kerry sounded a sombre note when he described the Syrian conflict as "in many ways out of control" after talks with UN and Arab diplomats in Geneva. He said there was progress in reducing violence in Aleppo where 250 people have been killed in the last nine days in after Syrian government air strikes and artillery attacks on rebel-held districts.
"We're trying to press this as fast as possible but I don't want to make any promises that can't be kept," he said.
Kerry said the attack on a hospital was "unconscionable" but both sides had contributed to "this chaos".