The leader of Lebanon's Hezbollah has warned the fall of Syrian president Bashar Assad's regime would give rise to extremists and plunge the Middle East into a "dark period".
He vowed his Shiite militant group will not stand idly by while its chief ally in Damascus is under attack.
In a televised address, Sheikh Hassan Nasrallah said Hezbollah members are fighting in Syria against Islamic extremists who pose a danger to Lebanon, and pledged that his group will not allow Syrian militants to control areas that border Lebanon.
Nasrallah's comments marked the first time he has publicly confirmed his men were fighting in Syria, and were his first remarks since Hezbollah fighters have become deeply involved in the battle for the strategic Syrian town of Qusair near the Lebanese frontier.
Hezbollah has come under harsh criticism at home and abroad for sending fighters to Syria to fight alongside Assad's forces. In his speech, Nasrallah sought to defend the group's deepening involvement, and frame its fight next door as part of a broader battle against Israel. He also portrayed the fight in Syria as an "existential war" for anti-Israel groups including Hezbollah.
"Syria is the back of the resistance, and the resistance cannot stand, arms folded while its back is broken," Nasrallah told thousands of supporters from a secret location though a video link.
"If Syria falls in the hands of America, Israel and the takfiris, the people of our region will go into a dark period," he said in a speech to mark the anniversary of Israel's withdrawal from southern Lebanon in May 2000. "If Syria falls, Palestine will be lost."
Earlier, fForces loyal to Assad unleashed their heaviest artillery barrage in a week to dislodge rebels from a strategic western town, according to activists. Pro-Assad troops, including fighters from the Lebanese militia Hezbollah, launched an offensive against the rebel-held town of Qusair a week ago. They have gained ground, but rebels continue to hold some positions.
Local activist Hadi Abdullah and the British-based Syrian Observatory for Human Rights say the artillery barrage began early on Saturday. The regime wants to recapture Qusair to open a land corridor between its strongholds in the capital Damascus and on the Mediterranean coast.
The Qusair offensive has highlighted Hezbollah's growing role in Syria's civil war. The latest push comes ahead of a speech by Hezbollah chief Hassan Nasrallah, his first since the offensive began.