The ascent of the extremists in the northwestern province of Idlib coincides with a suspension of aid to moderate rebel groups by their international allies.
The commanders of five of the groups say they were told earlier this month by representatives of the United States, Saudi Arabia and Turkey that they would receive no further arms or ammunition until they unite to form a coherent front against the jihadists, a goal that has eluded the fractious rebels throughout the six years of fighting.
The freeze on supplies is unrelated to the change of power in Washington, where the Trump administration is engaged in a review of U.S. policy on Syria, U.S. officials say. It also does not signal a complete rupture of support for the rebels, who are continuing to receive salaries, say diplomats and rebel commanders.
Rather, the goal is to ensure that supplies do not fall into extremist hands, by putting pressure on the rebels to form a more efficient force, the rebel commanders say they have been told.