Islamist rebels entered neighbourhoods in the regime-held city of Idlib in northwest Syria for the first time in four days of fierce clashes with government forces, a monitor reported.
"The armed groups have entered a number of areas inside the city and clashed violently with regime soldiers," said Rami Abdel Rahman, head of the Britain-based Syrian Observatory for Human Rights.
He said the Islamists had advanced significantly from the northwestern and southeastern sides of the city, and that the total death toll since Tuesday was 117.
The city is almost completely encircled by rebel groups, leaving only two exit routes for pro-regime forces.
An activist from the area said civilians were trapped in their homes.
"The humanitarian situation is very difficult. There are refugees from other parts of the country who are in Idlib," said Ibrahim al-Idlibi.
On Tuesday, a new coalition of Islamist rebels launched a coordinated attack against Idlib city, which is held by government forces.
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Calling itself "The Army of Conquest," the coalition is led by Al-Qaeda affiliate the Al-Nusra Front and includes several other Islamist groups.
The rebels have advanced using "street fighting," Khaled Dhanoun, who is in contact with rebels in Idlib, told AFP.
According to Abdel Rahman, the Syrian army had sent in reinforcements and carried out air strikes on rebel positions on the outskirts of Idlib.
The province of the same name borders Turkey and is largely under the control of Al-Nusra, but provincial capital Idlib city remains in regime hands.
If it falls to rebel groups, it would be only the second provincial capital lost by the regime after Raqa, which is now the de facto capital of the Islamic State jihadist group.
Since Syria's conflict began in March 2011, more than 215,000 people have been killed and around half the country's population has been displaced.