Syrian troops and Lebanese Shi'ite militias attacked rebel-held areas on the two countries' border on Sunday, in the heaviest clashes of Syria's civil war in the strategic region, Lebanese and Syrian sources said.
At least two towns held by Sunni Islamist rebels in the al-Qusair region near the Orontes River were overrun after sectarian clashes escalated early last week, threatening to bring in Iranian-backed Hezbollah openly into the battle, the sources said.
On Saturday rockets hit the town of Hermel, a Hezbollah stronghold in Lebanon's Bekaa Valley, causing damage but no casualties, and a Hezbollah fighter was killed in the Shi'ite border town of Zita inside Syria, residents said.
Six rebel fighters were killed in clashes in Qusair on Sunday and one woman was killed in Syrian air force bombing on the region, opposition campaigners said.
The border area, known for smuggling for decades, is an important supply line for rebels fighting President Bashar al-Assad's forces in the central city of Homs, a main front in the war.
The conflict started two years ago with peaceful demonstrations against four decades of rule by the Assad family, who belong to the Alawite sect, an offshoot of Shi'ite Islam.
At least 70,000 people have been killed in the conflict, which is increasingly pitting majority Sunnis against the minority Alawites, who have controlled Syria since the 1960s.
The Syrian war has worsened Lebanon's own sectarian tensions, with wounds from a 15-year civil war that ended in 1990 far from healed. Syrian maintained a 29-year military presence in Lebanon until it was forced to withdraw its troops under international pressure in 2005.
Assad, who has lost control of large parts of Syria, has been on the offensive in the centre and north of the country in the last few weeks.
In the border region, Syrian army troops and Shi'ite militias entered the towns Saqraja, which controls the approaches to the main rebel-held town of Qusair, and al-Radwaniya, while intense fighting was reported in the nearby village of Burhaniya, the sources said.
The official Syrian state news agency Sana said "the brave Syrian army spread its control of Saqraja after it destroyed the last remnants of the terrorists."
Speaking from Qusair, activist Hadi al-Abdallah said Hezbollah and its militia allies were pushing from the Bekaa valley toward Qusair, while the Syrian army is moving south from Homs in a pincer movement aimed at ending the rebel presence along the border.
"In the last few days there have been attacks by the Hezbollah forces on new villages around Qusair," he said, adding that Hezbollah and its allies had already taken eight villages and towns along the border inside Syria.
"This is part of a bigger strategy to control Homs and link it with the Bekaa and the coast," he said, referring to Alawite regions near the Mediterranean where the opposition suspects Assad will rebase and form an Alawite enclave if his position in Damascus becomes untenable.
"We do not want to reach a stage where the rebels would have to hit Lebanese territory at random," Abdallah said, adding that the opposition would hold their fire if the attacks on Qusair and the surrounding towns was called off.