A new Islamist rebel alliance, including Al-Qaeda's Syrian affiliate, was locked in a fierce battle Friday to seize government-held areas of Aleppo, the divided former economic capital.
Once a powerhouse of industry, Aleppo has been devastated by years of fighting between regime forces and a succession of rebel groups.
Clashes raged overnight as the Islamist alliance, which calls itself Ansar al-Sharia, sought to take control of the air force intelligence headquarters in Zahra, on Aleppo's northwestern outskirts, said the Syrian Observatory for Human Rights.
According to the British-based monitor, the 13 groups in the alliance announced the launch of the "Ansar al-Sharia operations room" on Thursday.
They said the aim was to "liberate Aleppo and the countryside" and "to draft a joint covenant to run Aleppo after its liberation in line with sharia" Islamic law.
The rebels advanced to take control of several buildings in Zahra despite regime air strikes, according to Observatory head Rami Abdel Rahman.
"There were at least 35 dead among insurgent ranks and dozens of killed and wounded on the regime side," he told AFP.
The coalition, which includes Al-Qaeda's Syrian associate Al-Nusra Front, the rival of the Islamic State jihadist group, pledged "victory for the Muslims of Aleppo".
Syrian state television said that the army had "foiled attempts to infiltrate Aleppo on several fronts, killing more than 100 terrorists" -- the regime's standard term for all rebel groups.
To the west of the city, rebels from a different Islamist-dominated alliance called Conquest of Aleppo captured parts of a military research centre, the Observatory said.
It said 11 rebels and an unknown number of regime troops were killed in that battle.
On Thursday, Ansar al-Sharia launched a multi-district assault on government-held parts of Aleppo city, in attacks that killed at least four civilians, the Observatory said.
Rebels fired several hundred rockets and projectiles into at least seven government-held neighbourhoods, with the army returning fire and regime aircraft carrying out raids.
Fighting resumed Friday before dawn on pro-government areas of the Ashrafiyeh and Khaldiyeh in the city's northern and western sectors, the monitor said.
Abdel Rahman said hundreds of shells fell on both government- and rebel-held areas of the city, in what he said was Aleppo's "worst night" since 2012.
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One Aleppo resident, a 23-year-old student who gave her name as Sahar, said fighting had been "intensive".
"We are used to the sound of explosions but yesterday (Thursday) there were so many. We heard the blasts but because they were coming from everywhere we didn't know where the shells were falling," she told AFP by telephone.
- City, province divided -
Control of Aleppo has been divided between government and rebel forces since shortly after fighting began there in mid-2012.
The regime largely controls the west of the city, with rebels from different factions present in the east.
The situation is largely reversed in the countryside surrounding the city, and both government and rebel forces have at times sought to encircle their opponents and besiege them.
Activist Karim Obeid said the Ansar al-Sharia coalition had targeted Aleppo's Zahra "because the (Syrian) army regularly bombs opposition-held locations from there".
Taking Zahra would help to open up rebel access to the border with Turkey, Obeid added.
In recent months, Al-Nusra has allied elsewhere with Islamist rebels to win large swathes of territory from the regime, particularly in the northwestern province of Idlib.
Overnight, anti-regime forces also attacked an army outpost at the entrance to Zabadani near Damascus, according to the Observatory.
Zabadani is around 20 kilometres (12 miles) north of the capital and was one of the first towns to fall to rebels in early 2012.
Following a fightback aided by Shiite militant group Hezbollah, the government has seized control of several towns and villages close to the Lebanese border and Zabadani is the last in rebel hands.
The Observatory said the army responded by dropping more than 40 barrel bombs -- crudely made, non-guided missiles -- on the town, after at least three rebels and five regime troops were killed in skirmishes.
More than 230,000 people have been killed in Syria since the country's conflict began in March 2011.