Syrian army bombing in the mountainous area along the border with Lebanon killed dozens of insurgents and helped regain hilly territory overlooking Hezbollah strongholds, the Lebanese group said on Thursday.
For years, hideouts in the Qalamoun mountains have allowed al Qaeda-linked insurgents to attack Syrian soldiers and fighters from Hezbollah, a staunch ally of Syrian President Bashar al-Assad, along miles of borderland in eastern Lebanon.
The Syrian military backed by Hezbollah fighters captured the strategically important territory known as Assal al-Ward on the Syrian side of the border, after retaking Qarna Heights, a Hezbollah official told Reuters, both in the 100-km (60-mile) Qalamoun range.
Some insurgents were from al Qaeda's Nusra Front, he said.
Hezbollah's leader Sayyed Hassan Nasrallah vowed on Tuesday that his forces and Syrian allies would clear the rebels out of the border region north of the capital Damascus that insurgents use as a main supply route for arms and fighters.
Although he did not disclose when his fighters would start a major assault in the Qalamoun area, his group said on Wednesday that its fighters had hit a gathering of militants on the Syrian side of the border, inflicting heavy casualties.
Lebanese officials have warned Iran-backed Hezbollah against launching a major cross-border attack, which they say would drag Lebanon, which suffered a civil war from 1975 to 1990, further into the years-long Syrian conflict.
On Wednesday, Hezbollah said its fighters had retaken part of a hilltop in Lebanon's eastern border area known as Kherbat al-Nahla overlooking the Syrian side of Qalamoun.
But rebel group Jaish al-Fatah Qalamoun said on social media sites they had repelled the assault which Hezbollah has been rumoured to have been planning for months.
In Syria, the army continued an offensive across rebel-held parts of the northwestern province of Idlib to regain significant amounts of ground lost in recent weeks.
The Syrian airforce intensified raids near a hospital on the outskirts of rebel-held Jisr al-Shughour where a number of Syrian troops have been holed up since insurgents captured the town last month. The raids prevented insurgents from making advances into the rich agricultural province by the Turkish border.
By taking Jisr al-Shughour, the rebels have edged closer to the coastal province of Latakia, one of the main government-held strongholds, and are now less than 8 km from pro-government villages near the sea.