Syrian troops regrouped on Sunday after a coalition including Al Qaeda's local affiliate seized the city of Idlib, the second provincial capital to fall from government control.

The capture is a blow to the government and raises the prospect that the city will become the effective capital of territory held by Al Qaeda's Syrian wing, the al-Nusra Front, analysts said.

On Sunday, 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.

The Al Watan newspaper, which is close to the government, said troops had carried out a "successful operation regrouping south of the city."

"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.

The coalition of Islamists overran Idlib on Saturday, after an operation that began five days earlier and killed at least 170 opposition and regime troops. The city becomes only the second provincial capital to fall from regime control after Raqqa, in northern Syria, which was seized by rebel groups in March 2013.

Those groups were ousted from the city by the Islamic State, which made Raqqa the de facto Syrian capital of its self-declared Islamic "caliphate" on Syrian and Iraqi territory.

The group that seized Idlib calls itself the Army of Conquest and includes al-Nusra and the powerful Islamist Ahrar al-Sham group, as well as other smaller conservative Islamist rebel forces.

They touted their victory on social media, with al-Nusra's Twitter accounts posting photos of its fighters in front of government buildings.