which was seized by rebels in March 2013.
The rebels were subsequently ousted by the Islamic State group, which has made Raqa the de factor Syrian capital of its self-proclaimed Islamic “caliphate” in Syria and Iraq.
The Observatory said street fighting had raged through the night after the rebels entered Idlib on Friday evening in an assault that began three days earlier.
Around 2,000 Al-Nusra and allied fighters took part in the battle, according to the Observatory, while the regime launched some 150 air strikes in a bid to hold off the attack.
At least 130 people were killed in the fighting, according to the Observatory, which said some of the city’s residents had fled. Around 200,000 people lived in the city before the conflict, but the population has swelled since with Syrians displaced from other areas.
Idlib province as a whole is a bastion of Al-Nusra, which ousted several rebel groups, including Western-backed organisations, from the region in November.
That came after the group announced plans to establish an Islamic “emirate” in the area, which analysts say is intended to rival IS’s “caliphate”.
Idlib’s fall leaves the regime with few remaining strongholds in the province, which borders Turkey.
Government forces are still present in the cities of Jisr al-Shugur and Ariha, a few small localities, the Abu Duhur military airport and five military bases.
Idlib’s capture came as the UN’s Ban spoke of his disappointment over the continuing death and destruction.
“I confess to you my anger and my shame,” he told an Arab summit in Egypt.
“Anger at observing the Syrian government, extremist and terrorist groups and terrorists relentlessly destroy their country,” he said.
“Shame at sharing in the collective failure of international and regional communities to decisively act to stop the carnage that has afflicted the Arab brothers and sisters of Syria.”