BEIRUT — Insurgents fired nearly 400 rockets at two Shiite villages in northwestern Syria on Friday and detonated at least nine car bombs, a monitor said, in a new assault on besieged government-held areas.

An alliance of insurgent groups, including al-Qaeda’s al-Nusra Front, attacked al-Foua and Kefraya in Idlib province, an area bordering Turkey that is mostly held by insurgents after rebel advances this year, the Syrian Observatory for Human Rights said.

Thousands of civilians have been living under siege in the two villages, which are still held by government forces.

Fierce clashes raged between the government forces and insurgents of the “Army of Conquest,” a coalition of groups that includes Nusra Front and the powerful Ahrar al-Sham, the Observatory said. Seven of the nine car bombs were suicide attacks, it added.

At least 17 fighters were killed in clashes, separate from the suicide attacks, the Observatory said.

Syrian state TV said forces on the government side had prevented several car bombs from reaching their targets and destroyed them, also killing rebel fighters.

Warring sides agreed to a brief ceasefire last month in the two villages and in Zabadani, a rebel-held town near the Lebanese border under siege by government forces and the Lebanese Shiite group Hezbollah, an ally of Damascus.

But the two-day ceasefire, the second that month, collapsed before the wounded could be evacuated from either area.

Government forces have since intensified their assault on Zabadani and say they are on the verge of seizing the town.