Leaders from ISIS and al-Qaeda have signed a truce, and have agreed to work together to fight their opponents, AP reports.
According to a top Syrian opposition official and a rebel commander, the terrorist forces will combine to counter the US-led coalition which is engaging in airstrikes across the region.
A meeting on November 2 concluded that infighting would stop, and militants from each group would come together against Kurdish fighters in northern Syria.
For over a year, ISIS and al-Qaeda's Syrian affiliate, Jabhat al-Nusra, have been fighting one another, each hoping to lead the rebellion against President Bashar al-Assad.
Led by Abu Bakr al-Baghdadi, ISIS was originally formed out of al-Qaeda in Iraq in April 2013, and its core is made up of former al-Qaeda militants. However, a rift between the two groups led to a severing of ties in early February this year.
Associate professor of political science at Haverford College, Barak Mendelsohn, at the time attributed this rift to ISIS' "disrespect and even outright subversion of al-Qaeda's authority".
Al-Baghdadi had apparently claimed to command Jabhat al-Nusra and declared that the two would operate under the name ISIS.
A cooperation deal between the groups, if true, will mean rebel factions mobilised by Washington to halt ISIS' advance will face a difficult task. "Those rebels, already considered relatively weak and disorganized, would face far stronger opposition if the two heavy-hitting militant groups now are working together," AP reports.
However, it is thought that any truce made by al-Qaeda and ISIS could easily break down, and it is unlikely that the two will officially unite.
Despite this, an American analyst warned: "If there is less blood being spilled against each other and they don't have to worry about that, that's going to make it easier for the jihadis to go after Assad or any Western-backed forces".
The news follows an audio message released by al-Baghdadi, in which he urges his followers to unleash a "volcano of jihad" upon the West.
Washington is now considering whether to deploy troops on the ground to combat the Islamic extremists. General Martin E Dempsey, Chairman of the Joint Chiefs, said yesterday: "I am not predicting at this point that I would recommend that [Iraqi] forces in Mosul and along the border would need to be accompanied by US forces, but we're certainly considering it."
President Barack Obama has previously repeatedly stressed that there will be no active combat on the ground, saying: "I will not allow the United States to be dragged into another war in Iraq."