Such an accord could present new difficulties for Washington's strategy against the IS group. While warplanes from a 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 disorganized, would face far stronger opposition if the two heavy-hitting militant groups now are working together.
The Associated Press reported late last month on signs that the two groups appear to have curtailed their feud with informal local truces. 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.
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A US official with access to intelligence about Syria said the American 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. The official insisted on anonymity because he said he was not authorized to speak publicly about the subject.
The official said the meeting was closely followed by members of his movement, and he is certain that an agreement was reached. The official 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 had learned that high-ranking members of Nusra and IS met on November 2.