AMMAN (Reuters) - The Syrian army, backed by Russian air strikes, fought heavy battles with insurgents for control of a hilltop close to President Bashar al-Assad's coastal heartland, a monitor and rebels said on Thursday.

The fighting was centered around Jub al Ahmar, a highland area in Latakia province which if captured would allow the army to more effectively pound rebel positions from the Ghab Plain that it overlooks, said Rami Abdulrahman, the head of the Syrian Observatory for Human Rights.

"Fighting has been intensifying in the vicinity of the strategic Jub al Ahmar heights with some progress by the army but no decisive gains so far," Abdulrahman told Reuters.

The army announced on Thursday it had extended a major offensive to the Plain area whose capture from rebel groups, including al Qaeda's Nusra Front would help secure Assad's coastal heartlands and could provide a platform to drive the rebels back from other areas.

The rebels thrust into the area in late July after seizing much of the northwestern province of Idlib.

Rebel sources also said the army aided by Russian jets shelled the insurgent held town of Salma, which located northeast of Latakia and is a major frontline between rebels and government forces.

"This area is shelled daily but we have not seen this intensity before with heavy mortars and artillery and Russian bombing of our positions," said Abu Ibrahim al Hifnawi, a rebel from Ahrar al Sham insurgent group.

(Reporting by Suleiman Al-Khalidi; Editing by Alison Williams)