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.
Al-Nusra also posted video of its forces entering a prison in the city, where they discovered the bodies of detainees apparently executed by government forces before their withdrawal.
The Observatory said at least 15 prisoners had been found dead at a military intelligence detention facility in the city.
The city’s capture was praised by opposition forces across the spectrum of Syria’s uprising, which began in March 2011 with anti-government protests before descending into a brutal war after a regime crackdown.
The opposition National Coalition, which is recognised by much of the international community, welcomed it as “an important victory on the road to the full liberation of Syrian territory”.
Its statement made no reference to the composition of the forces that seized the city, saying only it had “confidence” they would protect civilians and abide by international law.
Analysts said Al-Nusra’s role in Idlib’s capture had put the Coalition and other “moderate” rebels in a difficult position.
“The capture of Idlib is huge boost for the Syrian opposition, but it’s one that once again will serve to underline the relative inadequacies of genuine ‘moderates’” said Charles Lister, a visiting fellow at the Brookings Doha Centre think-tank.
Al-Nusra and its allies already control large portion of Idlib province, after a November offensive in which they ousted several Western-backed opposition groups.
And last year, the group announced plans to eventually establish an “emirate” that analysts say is intended to rival IS’s “caliphate”.
“Considering the posture established by Al-Nusra in Idlib over the last nine months, it seems pretty implausible that the city won’t eventually end up representing Nusra’s effective capital and stronghold,” Lister said.
More than 215,000 people have been killed in Syria since the conflict began, and half the country’s population has been displaced in the war.