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BEIRUT (Reuters) - 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."

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 may give him the upper hand in proposed U.S.-Russia led peace talks next month.

Syria's two-year uprising against four decades of Assad family rule began as peaceful protests but devolved into an armed conflict that has killed more than 80,000 people.

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.

A fighter from Hezbollah forces in Qusair told Reuters that advances were happening at a very slow pace.

"We are in the second phase of our plan of attack but the advance has been quite slow and difficult. The rebels have mined everything, the streets, the houses. Even the refrigerators are mined."

The fighting in Qusair has also highlighted the 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, which fought its own sectarian-fuelled 15-year civil war, has seen a rise in Syria-linked violence.

Syria's Sunni Muslim majority has led the struggle to topple Assad, and 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 regional Shi'ite power Iran, a longtime ally, and now increasingly by the country's Lebanese proxy, Shi'ite Hezbollah, founded as a resistance movement to Israel.

Syrian rebels now say that whatever the outcome, they will plot sectarian revenge attacks on Shi'ite and Alawite villages on either side of the border.

The Syrian Observatory for Human Rights, a Britain-based opposition monitoring group with a network of activists across Syria, said Assad forces led by Hezbollah were trying to advance from three directions in the city.

"Every area they didn't have a foothold in, they are trying to gain one now," Rami Abdelraham, head of the Observatory, told Reuters by telephone.

The violence in Qusair, part of Syria's central Homs province, has sparked clashes in the nearby Lebanese city of Tripoli.

More than 25 people have been killed in seven days of fighting in Tripoli, one of the bloodiest episodes of Syria-related fighting in the coastal city. Both sides of the conflict accuse the other of using Tripoli as a part of the supply chain over the border into Syria.

Rebels from across Syria say they have sent some of their units into Qusair.

Colonel Abdeljabbar al-Okaidi, the Aleppo-based regional leader of a moderate, internationally-backed Supreme Military Council said he and the 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 yet and insisted the rebels locked in Qusair were still on their own.

"No one is helping Qusair other than its own men," he said.

(Reporting by Erika Solomon; Additional reporting by Laila Bassam; Editing by Sonya Hepinstall)