* Attack expands militants' presence in west

* 50 fighters killed or wounded on government side

* Islamic State within 20 km of important highway

BEIRUT, Nov 1 (Reuters) - Islamic State fighters seized the Syrian town of Maheen in Homs province from government forces on Sunday, a monitoring group said, expanding their presence in Syria's west despite a Russian-backed bombing campaign against them.

Islamic State's strongholds in Syria are in the north and east, but it has increased its territory in Homs province since taking over the historic city of Palmyra earlier this year, and then Qaryatain, 15 kilometres east of Maheen.

The group began its attack late on Saturday using two suicide car bombs and by Sunday morning had taken over Maheen, the Britain-based Syrian Observatory for Human Rights said.

The assault brought Islamic State to within 20 kilometres (13 miles) of the main highway that links Damascus to Homs and to cities further north.

The Observatory said at least 50 fighters on the government side were killed or wounded, and that clashes were raging further west on the outskirts of Sadad, a nearby town mostly inhabited by Christians, as Islamic State pressed its advance.

A statement from Islamic State confirmed the assault on Maheen, describing the town as "strategically important" and saying it had also seized weapons caches.

Russia and the Syrian government have been waging an air offensive against insurgents in the west and northwest of Syria.

The Observatory's Rami Abdulrahman said the attack might have been a response to pressures the group is under elsewhere.

"Daesh always looks for advances against the regime after failures in the areas it controls in northern Syria," he said, without referring to specific battles in the north, and using an Arabic name for Islamic State.

Government and Russian air strikes have been targeting IS fighters at an airbase they have long besieged in Aleppo province. Fighting there has drawn support from Damascus ally Iran and killed an Iranian general several weeks ago. .

Islamic State is also under pressure in the northeast province of Hasaka, after a new U.S.-backed rebel alliance launched an offensive against the group near the Iraqi border. (Reporting by John Davison; Editing by Raissa Kasolowsky)

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