Fierce clashes raged at the eastern entrance to the Syrian city of Idlib as regime forces battled a coalition of rebels led by Al-Qaeda's local branch on Wednesday, a monitor said.
The clashes came a day after a new coalition made up of Al-Qaeda affiliate Al-Nusra Front and its Islamist allies launched an operation against the regime-held provincial capital in northwest Syria.
"Fighting is ongoing at the eastern entrance and in the outskirts of the city, which is also being shelled by Al-Nusra Front and its Islamist allies," said Rami Abdel Rahman, head of the Syrian Observatory for Human Rights monitoring group.
Idlib province is largely under the control of Al-Nusra, but the provincial capital Idlib city remains in regime hands.
If it falls, it would be only the second provincial capital to fall from regime control after Raqa, which is now the de facto capital of the Islamic State jihadist group.
At least 40 people have been killed in the fighting since Tuesday, among them 20 regime forces and 20 fighters from the Islamist groups Ahrar al-Sham and Al-Nusra, the Observatory said.
"The Islamist group Ahrar al-Sham regained control of four checkpoints in the suburbs of the city," Abdel Rahman said.
The opposition forces carried out three suicide bomb attacks against regime checkpoints outside the city, the Observatory said.
Al-Nusra said on Twitter that dozens where killed when it attacked a building used by regime forces with a military vehicle laden with explosives.
Syria's SANA state news agency reported "fierce clashes on the outskirts of Idlib city between army units and terrorist groups including Al-Nusra Front... that tried to launch a terrorist attack on the outskirts of the city yesterday."
"The army inflicted heavy losses," the agency said, reporting "dozens" of rebel deaths.
Idlib governor Khaireddin al-Sayyed told AFP the army was backed by the National Defence Forces and Popular Committees, pro-government militia groups.
"They will not advance an inch into the city," he told AFP by phone.
"There is no evacuation plan because the residents will not be leaving, they will participate in defence of the city," he added.
Sayyed said Idlib's pre-war population of around 200,000 had swelled massively since Syria's conflict began in March 2011, with waves of displaced people taking refuge in the city.
Al-Nusra controls much of Idlib province after it chased out rival groups, including Western-backed opposition fighters, last November.
Government forces control Idlib city, along with the towns of Jisr al-Shughur and Ariha, Abu Duhur military airport and five military bases in the province.
More than 215,000 people have been killed in Syria's four-year conflict and around half the country's population has been displaced.