Islamic State fighters launched an attack on Syrian army-held areas in Homs province overnight, a Syrian military source and an organization tracking the war said, part of the group's efforts to expand beyond its strongholds in the east.

Syrian troops repelled the attack in places and were still fighting in others, the military source said. The British-based monitoring group, the Syrian Observatory for Human Rights, said there were large numbers of dead but had no toll.

The Islamic State's (ISIS) targets included the town of al-Sukhna, some 220 kilometers, 140 miles, east of Homs city, the Observatory said. The military source said that the army "repelled the attack in areas, there are areas where it is still ongoing."

ISIS has in recent months launched attacks in both the Syrian government-held as well as rebel-held areas far beyond areas of northern and eastern Syria where it is most firmly entrenched.

The attacks have added to pressure on the army and allied militia that have faced significant setbacks recently, notably in the northwestern province of Idlib where rebels have seized the provincial capital and the nearby town of Jisr al-Shughour.

The Lebanese group Hezbollah, a crucial ally of President Bashar al-Assad, and the Syrian military have meanwhile launched a big push to drive insurgents from the strategically important border zone between Lebanon and Syria.

Media run by Hezbollah, an Iranian-backed Shiite group with a powerful militia, on Wednesday reported further progress in the offensive to drive insurgents including members of the al Qaida-linked Nusra Front from the Qalamoun mountain range.

The offensive is focused on areas about 50 kilometers north of Assad's seat of power in Damascus that are seen as a top priority as he and his allies try to shore up control over over heavily populated parts of western Syria.

Hezbollah's media arm, al-Manar, said the group's fighters and the Syrian army were bombarding a hill known as Tel Moussa — seen as the most important target in the offensive due to its high elevation overlooking the entire area.