The assault brought the group to within 20 kilometres (13 miles) of the main highway that links Damascus to Homs and cities further north, even as Russia and the Syrian government intensified a bombing campaign against insurgents in the west and northwest of the country.
Islamic State began the 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 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, describing the town as "strategically important" and saying it had also seized weapons caches.
Islamic State's strongholds in Syria are in the north and east of the country, but it has increased its presence in Homs province since taking over the historic city of Palmyra earlier this year, and then Qaryatain, 15 kilometres east of Maheen.