An alliance of Syrian Islamist rebels including al Qaeda's Nusra Front have overrun 17 defence posts around Idlib in an offensive to take the city from the army and allied militia, a monitoring group said on Thursday.
Idlib, which is close to the main strategic highway linking Damascus to Aleppo, is the capital of a northwestern province of the same name. The army controls the city of at least 100,000 people and rebels are stationed on the outskirts.
The Syrian Observatory for Human Rights, which tracks the violence from Britain, said the Nusra Front, the powerful Ahrar al-Sham movement and the hardline Jund al-Aqsa were advancing, taking a total of 17 army posts on the outskirts.
It said 11 soldiers and allied militia as well as 17 insurgents were killed on Wednesday and violent clashes continued on Thursday.
Syria's state news agency said on Thursday that government forces had "targeted terrorist gatherings in the surroundings of Idlib city". It said dozens of insurgents were killed and 11 vehicles were destroyed.
State television said the army foiled an attempt by terrorists to enter the city, citing a military source.
The grouping of Islamist brigades have called the operation Army of Fatah, a reference to the Muslim conquests that spread the faith through the Middle East starting in the seventh century.
The Islamic Front, an alliance involved in the operation, posted videos on YouTube of battlefield developments on Thursday. It showed insurgents in camouflage running through olive groves.
The Islamic Front's Twitter account said that a top military leader in Ahrar al-Sham had been killed during the offensive. Islamic State, the most powerful anti-government militia, was not involved in the clashes.
Syria slid into civil war after a crackdown on peaceful protests against President Bashar al-Assad in March 2011. The United Nations says more than 220,000 people have been killed.