The leader of the Lebanese Shia militant Hezbollah movement, Hassan Nasrallah, has promised his supporters of victory in Syria, where they are backing President Bashar al-Assad.
Syrian rebels in the besieged town of Qusair say they are under heavy bombardment from Hezbollah combatants. Qusair is important for the Syrian government because it links the capital, Damascus, with the Alawite heartland on the Mediterranean coast, reports BBC News.
Hasrallah said in a speech from an undisclosed location that if Sunni Islamists took over in Syria, they would pose a threat to the entire Lebanese population - Shia and Sunni Muslims, as well as Christians.
He said his movement could never be aligned with Syrian rebels who, in his view, were supported by the US and Israel.
Dozens of Hezbollah militants are said to have been killed fighting alongside Syrian troops in Qusair since May 19, when government forces launched an offensive to recapture the rebel-held town.
Last week, US Secretary of State John Kerry said thousands of Hezbollah fighters were contributing significantly to the violence in Syria. He added that Iran was actively supporting Hezbollah's involvement - a claim denied by Tehran.
Iran and Hezbollah are predominantly Shia, while Assad's Alawite sect is an offshoot of Shia Islam.
Syrian state media said the army had launched a three-pronged offensive in the north, centre and south of Qusair, and was making big advances after "killing large numbers" of fighters.
The Lebanese group is also known to have lost a number of fighters in Qusair, prompting Lebanese President Michel Suleiman to warn the Shia militia against getting "bogged down in the sands of discord".
The Syrian Observatory for Human Rights, a UK-based activist group that monitors the conflict, said at least 22 people including 18 rebels had been killed in the latest fighting in Qusair.
