A wave of air strikes by government ally Russia killed at least 21 Islamic State group militants in the Syrian desert over the past 24 hours, a monitor said Saturday.

A car bomb exploded in the rebel-held Syrian city of Idlib on Saturday, a war monitor and opposition news channel said, as air strikes hit its outskirts in a government offensive on the last major opposition bastion.

Syrian troops on Friday reclaimed a cluster of towns they had lost early in the eight-year-old war, pressing their offensive in the northwest, the country's last big rebel stronghold.

Syrian rebels withdrew from a town in southern Idlib province and from their last remaining territory in neighbouring Hama province after government troops advances in the area

Regime loyalists captured a town and a village from militants and allied rebels in northwest Syria Wednesday, after fierce clashes that killed nearly 30 combatants overnight, a war monitor said.

Syrian government forces have recovered two villages from rebel fighters in northwestern Syria after days of bombardment, sources on both sides said on Monday.

Regime bombardment Sunday killed 12 civilians including three children in an opposition bastion in northwestern Syria, a war monitor said.

Nearly 100 combatants were killed on Friday in clashes between fighters loyal to Syria's government, rebels and jihadists in northwestern Syria, a war monitor said.

Attacks by Syrian regime forces and their Russian allies killed 12 civilians in the country's northwest on Friday in the latest violation of an eight-month-old truce, a war monitor said.

An Israeli airstrike on a military base in Syria's Hama province early Saturday injured three people, Syrian state media reported.

Syrian government forces fought off an attack by a group of suicide bombers on a position in western Syria on Tuesday, during which three people were killed in addition to all of the militants, a state-run TV station reported.

At least 15 people were reported killed on Sunday in shelling by government and insurgent forces in northwestern Syria, further straining a Russian-Turkish ceasefire deal for the region.