Al-Qaeda's Syria affiliate seized control of most of Idlib on Saturday after government forces pulled back to bases in the heart of the main northwestern city, a monitoring group said.
Al-Nusra Front and its Islamist allies "control the majority of neighbourhoods in Idlib, apart from government and security buildings," Rami Abdel Rahman, director of the Syrian Observatory for Human Rights, told AFP.
A Syrian security source acknowledged that "terrorist groups have infiltrated the outskirts of the city" but said troops were battling to push them back.
The Observatory said street fighting had raged through the night after the rebels entered the government-held city on Friday evening.
More than 130 people have been killed since fighting around the city began on Tuesday, the Britain-based group said.
Idlib province, which borders Turkey, is a bastion of Al-Nusra, which has driven out several rival rebel groups.
Government control is restricted to Idlib city, the towns of Jisr al-Shughur and Ariha, the Abu Duhur air base and five army garrisons.
Were the jihadists to take Idlib, it would become only the second provincial capital after Raqa -- the stronghold of the Islamic State group -- to fall out of government hands since the conflict erupted in 2011.
Video footage posted by Al-Nusra showed fighters ripping down a poster of President Bashar al-Assad and trampling it under foot in what it said was a captured neighbourhood of Idlib.
Another posting showed what it said were fighters from the city returning to their homes for the first time since the conflict erupted in 2011.
"This is my home, this is our neighbourhood, our land. God willing, we will establish Islam," one says to the camera.
On Tuesday, a new coalition of Islamist rebels launched a coordinated attack against Idlib.
Calling itself "The Army of Conquest," the coalition is led by Al-Nusra and includes several other groups.