Current and former U.S. officials say they are unaware of any cooperation between ISIS and al Nusra, and they doubt that a merger or long-term association could be pulled off. “I find it hard to believe that al Nusra and Islamic State could sink their differences,” says a former senior administration official. “The rift between them is very deep,” he adds.

But senior Syrian opposition sources say efforts at a merger are very much under way and they blame Washington for creating the circumstances that make it possible. Moderate rebels accuse the Obama administration of fostering jihadi rapprochement by launching ill-conceived airstrikes on al Nusra while at the same time adamantly refusing to target the forces of Syrian President Bashar al-Assad in the U.S. military intervention in the region.

This, they say, has created the opening for a possible understanding between the jihadists and is creating sympathy for al Nusra. Other Islamist rebels and the wider population in insurgent-held areas in northern Syria question American motives and designs and remain furious at the U.S. decision not to help topple Assad.

“Al Nusra knows more airstrikes are coming, so why wait,” says an opposition source.