Syrian troops and their allies, backed by Russian jets, attacked rebel-held towns north of the city of Homs, targeting a long-held and strategic enclave of opposition to President Bashar al-Assad.

The offensive that began before dawn builds on over a week of ground attacks launched with Russian air support in areas of western Syria that are crucial to Assad's survival and held by rebel groups other than Islamic State.

Syrian state television, quoting a military source, said the army had begun a military operation in the area after heavy air strikes and artillery barrages early yesterday.

The Syrian Observatory for Human Rights, a UK-based group which monitors Syria's four-year-old civil war, said at least five civilians and six insurgents had been killed in Teir Malla, about three miles (5 km) north of Homs city. A resident said at least 25 people were killed including, Rawad al Aksah, a commander of insurgent group Liwaa al-Tawhid.