An alliance of Syrian Islamist rebels including al Qaeda's official wing said they had launched an offensive on Tuesday to take the northwestern city of Idlib, which is close to the main strategic highway linking Damascus to Aleppo.
Districts of Idlib city, where more than 100,000 people live, have been controlled by both the army and insurgents since the crisis started four years ago. The army now controls urban areas and rebels are stationed outside the city.
A joint statement by insurgent groups including al-Qaeda's Nusra Front and the powerful Ahrar al-Sham movement said they would take the provincial capital.
The Syrian Observatory for Human Rights monitoring group, which tracks the violence from Britain, said the alliance had launched attacks on several areas of Idlib including army outposts but had not yet breached the edge of the city.
Syrian state television said the army had attacked militants in Idlib province but did not say there was an offensive to take the city. 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.