Militants of the Islamic State group and Jabhat al-Nusra, al Qaeda’s Syrian affiliate, struck a deal early in November to unite against the moderate, U.S.-backed rebels and Kurdish forces in Syria, according to a report by The Associated Press, or AP. Although a formal merger of the rival groups is not expected to be announced any time soon, the alleged truce between the two could result in a far stronger opposition to the U.S.-led campaign against ISIS in Syria.

According to a Syrian opposition official, quoted by AP, the truce was agreed upon in a meeting between senior militants of ISIS and al-Nusra on Nov. 2. The meeting, held in the town of Atareb -- nearly 20 miles west of Aleppo -- was also attended by militants of the Khorasan Group, al Qaeda militants from Afghanistan and Pakistan, and members of the Syrian rebel group Ahrar al-Sham.

During the meeting, an accord was reached to temporarily halt infighting between al-Nusra and ISIS, and to carry out joint attacks against Kurdish forces and rebels of the Syrian Revolutionaries Front -- who are armed and trained by the U.S. -- according to the AP report.

Al-Nusra and the Islamic State group have fiercely attacked each other for over a year in an attempt to take the lead in the fight against Syrian President Bashar Assad. An alliance between the two major militant groups in Syria could pose fresh difficulties for the U.S., which has so far relied on an offensive combining airstrikes and providing arms and training to moderate rebels. 

The moderate factions in Syria, who are already considered weak and disorganized, would now face a much stronger opposition from a joint front of al Qaeda and ISIS, AP reported.

An unnamed U.S. official familiar with the conflict in Syria told AP that though there is no credible information regarding a major shift in the strategy of the two militant groups, a local truce and “tactical deals on the ground” were definite possibilities.

Meanwhile, fresh American airstrikes on Thursday targeted strongholds of the al Qaeda-linked Khorasan Group in Syria for the third time since the start of the aerial campaign in the region. Although details of the latest attack have not been released, a spokesperson for the United States Central Command confirmed that an air raid targeting "a network of veteran al Qaeda operatives" had been carried out, according to a report by Agence France-Presse.