* At least 30 killed in opposition areas
* Assad's forces try to carve out route to coast (Updates death toll)
Syrian government forces and the Lebanese guerrilla group Hezbollah launched a fierce campaign to seize more rebel territory in the border town of Qusair on Saturday, sources on both sides of the conflict said.
Rebels fighting to topple President Bashar al-Assad said additional tanks and artillery had been deployed around opposition-held territory in Qusair, a Syrian town close to the Lebanese border.
"I've never seen a day like this since the battle started," said Malek Ammar, an activist speaking from the town by Skype. "The shelling is so violent and heavy. It's like they're trying to destroy the city house by house."
At least 30 people were killed in opposition-held areas on Saturday, most of them rebels, and dozens were injured, according to the Syrian Observatory for Human Rights.
Rebels are largely surrounded in Qusair, a town of 30,000 that has become a strategic battleground. Assad's forces want to take the area to secure a route between the capital Damascus and his stronghold on the Mediterranean coast, effectively dividing rebel-held territories in the north and south.
The opposition has been fighting back, seeing it as critical to maintain cross-border supply routes and stop Assad from gaining a victory they fear would give him the upper hand in proposed U.S.-Russia led peace talks next month.
Assad's forces are believed to have seized about two-thirds of Qusair, but the price has been high and rebels insist they are preventing any further advances.
"We are in the second phase of our plan of attack but the advance has been quite slow and difficult," an official close to Hezbollah told Reuters.
"The rebels have mined everything, the streets, the houses. Even the refrigerators are mined."
Hezbollah's Manar TV said the Syrian army had recaptured the Dabaa airport near the town, which rebels had seized several weeks ago.
There is an increasingly sectarian tone of Syria's political struggle, which is not only overshadowing the revolt but threatening to destabilize the region. Israel has launched two air strikes in Syria, and Lebanon has seen a rise in Syria-linked violence.
Syria's Sunni Muslim majority has led the struggle to topple Assad, which has been joined by Islamist fighters across the region, some of them linked to the militant group al Qaeda.
Assad comes from the minority Alawite sect, an offshoot of Shi'ite Islam, and has relied on an army led mostly by Alawite forces. He has been bankrolled by Shi'ite power Iran and now increasingly by Iran's Lebanese proxy, Shi'ite Hezbollah.
The British-based Observatory, which has a network of activists across Syria, said Assad forces led by Hezbollah were trying to advance from three directions in Qusair.
"Every area they didn't have a foothold in, they are trying to gain one now," said Rami Abdelraham, head of the Observatory.
Rebels from across Syria say they have sent units into Qusair.
Colonel Abdeljabbar al-Okaidi, the Aleppo-based regional leader of the moderate, internationally-backed rebel Supreme Military Council said he and Islamist brigade al-Tawheed had sent forces to the outskirts of the town to help the Qusair fighters.
But activist Malek Ammar said no forces had arrived and insisted the rebels surrounded in Qusair were on their own.
"No one is helping Qusair other than its own men," he said.