Militant leaders from Islamic State (IS) and al Qaida have agreed on a plan to stop fighting each other and work together against their opponents, a high-level Syrian opposition official and a rebel commander have said.
The accord could present new difficulties for the west's strategy against the IS group.
While warplanes from the US-led coalition strike militants from the air, the Obama administration has counted on arming "moderate" rebel factions to push them back on the ground.
Those rebels, already considered relatively weak and disorganised, would face far stronger opposition if the two heavy-hitting militant groups are working together.
IS, which has seized nearly a third of Syria and Iraq with a campaign of brutality and beheadings, and al Qaida's affiliate in Syria, known as the Nusra Front, have fought each other bitterly for more than a year to dominate the rebellion against Syrian president Bashar Assad.
Their new agreement, according to the sources in rebel groups opposed to both IS and Nusra Front, would involve a promise to stop fighting and team up in attacks in some areas of northern Syria.
Cooperation would fall short of unifying the rival groups, and experts believe any pact between the two sides could easily unravel.
US intelligence officials have been watching the groups closely and say a full merger is not expected soon - if ever.
An American official with access to intelligence about Syria said the US intelligence community has not seen any indications of a shift in the two groups' strategy, but added that he could not rule out tactical deals on the ground.
According to a Syrian opposition official speaking in Turkey, the meeting took place on November 2 in the Syrian town of Atareb, west of Aleppo, starting at around midnight and lasting until 4am.
The official said the meeting was closely followed by members of his movement, and he is certain that an agreement was reached. He said about seven top militant leaders attended.
A second source, a commander of brigades affiliated with the Western-backed Free Syrian Army who is known as Abu Musafer, said he also learned that high-ranking members of Nusra and IS met on November 2.
According to Abu Musafer, two decisions were reached: first, to halt infighting between Nusra and IS, and second, for the groups together to open up fronts against Kurdish fighters in a couple of new areas of northern Syria.
The Nusra Front has long been seen as one of the toughest factions trying to oust Assad in a civil war estimated to have killed more than 200,000 since 2011.
IS, also known as Isil and Isis, entered the Syria war in 2012 from its original home in Iraq and quickly earned a reputation for brutality and for trying to impose itself as the leading faction in the rebellion behind which all pious Muslims should unite.
Al Qaida initially rejected IS's claims to any role in Syria, and Nusra and other factions entered a war-within-a-war against it. But the Islamic State group swelled in power and became flush with weapons and cash after overrunning much of northern and western Iraq over the summer.
According to the opposition official, the meeting included an IS representative, two emissaries from Nusra Front, and attendees from the Khorasan Group, a small but battle-hardened band of al Qaida veterans from Afghanistan and Pakistan. Also reported to be present at the meeting was Jund al-Aqsa, a hard-line faction that has sworn allegiance to IS, and Ahrar al-Sham, a conservative Muslim rebel group.
The official said IS and the Nusra Front agreed to work to destroy the Syrian Revolutionaries Front, a prominent rebel faction armed and trained by the United States and led by a fighter named Jamal Maarouf. They agreed to keep fighting until all of the force, estimated to be 10,000 to 12,000 fighters, was eliminated, the official said.
During the meeting, IS also offered to send extra fighters to Nusra Front for an assault it launched last week on Western-backed rebels from the Hazm Movement near the town of Khan al-Sunbul in northern Syria, the official said.
IS sent about 100 fighters in 22 pickup trucks but Nusra ended up not needing the assistance, he said, because Hazm decided not to engage in the fight. Sixty-five Hazm fighters defected to Nusra, he said.