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

The offensive that began before dawn builds on more than 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 ISIS.

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 on Thursday.

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 3 miles 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.

A few miles further north, there were heavy air strikes around the town of Talbiseh and other villages in the area, the Observatory said, as well as fierce clashes on the southern edges of the town and nearby villages.

Recapturing the area north of Homs would help reassert Assad’s control over the main population centers of western Syria and secure territory linking Damascus to the coastal heartland of his minority Alawite sect.

“The regime is actually getting its forces ready in all Homs’s northern countryside … What we fear is that they will follow the same strategy they had in Hama countryside. They actually attacked the fighters in all fronts at once,” said local media activist Hassan Abou Nouh.

The Syrian army, supported by foreign allies, including Iran, has launched several ground offensives to retake insurgent-held territory since Russian jets started air strikes against rebel targets – mainly in western Syria – two weeks ago.