The Syrian army, backed by Hezbollah fighters, has made advances in mountainous areas along Syria's border with Lebanon, a Hezbollah official said on Friday.
The push in the Qalamoun mountain range appeared to be preparation for a wider offensive in the border region previously announced by Hezbollah, the staunch Lebanese ally of Syrian President Bashar al-Assad. Syrian state media said dozens of insurgents had been killed.
Hideouts in the Qalamoun mountains, north of the capital Damascus, have long allowed al Qaeda-linked insurgents to attack Syrian soldiers and Hezbollah fighters in the borderlands along Lebanon's eastern flank.
On Friday, a Hezbollah source said the army and Hezbollah were in control of around 100 sq km (40 sq miles) of the Qalamoun mountains by late on Thursday after advances in the strategic areas of Assal al-Ward and Qarna Heights.
The British-based monitoring group the Syrian Observatory for Human Rights said fighting had continued into Friday.
Hezbollah said in a statement that three of its fighters had been killed in the battles that started on Thursday, rejecting Lebanese media reports that more had died. A Twitter account from an alliance of Islamist insurgent groups including the Nusra Front, al Qaeda's Syrian wing, said scores of Hezbollah fighters had been killed.
The Hezbollah source said dozens of Nusra Front fighters had been killed, and large amounts of vehicles and weapons had been captured or destroyed. The Syrian state news agency SANA also said dozens had been killed.
Hezbollah's leader Sayyed Hassan Nasrallah vowed on Tuesday that his forces and their Syrian allies would clear the rebels out of the border region, which insurgents use as a main supply route for arms and fighters.
He did not specify when the major assault would start.
Some Lebanese officials have warned the Iranian-backed Hezbollah against launching a cross-border assault, saying it could drag Lebanon, which suffered its own civil war from 1975 to 1990, further into the Syrian conflict.