The capture is a blow to the government and raises the prospect that Idlib will become the effective capital of territory held by Al-Qaeda's Syrian wing, Al-Nusra Front, analysts said.
Today, the city in northwestern Syria was largely quiet, after sporadic government aerial bombardment overnight, the Britain-based Syrian Observatory for Human Rights monitor said.
A security source in Damascus and Syrian media said government forces were regrouping outside the city.
"Forces are repositioning on the outskirts of Idlib in order to face the terrorist battalions... And be in the best position to repel their attack," a security source in Damascus told AFP.
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"Army reinforcements were sent to start a military operation to regain control of the areas that were vacated after the evacuation of the local population to safe areas," the daily added, citing a source on the ground.
A coalition of Islamist forces overran Idlib yesterday, after an operation that began just five days earlier and killed at least 170 opposition and regime forces.
The city becomes only the second provincial capital to fall from regime control after Raqa, in northern Syria, which was seized by rebel groups in March 2013.