Syrian elite forces and extra fighters from the Lebanese Shiite Hezbollah movement have been sent to reinforce government troops battling rebels in the strategic border town of Qusayr, a watchdog said on Wednesday.

Government fighter jets early Wednesday bombed rebel zones of the town as regime forces readied to launch a major assault, according the Syrian Observatory for Human Rights.

Hezbollah fighters and crack troops of Syria's elite Republican Guards had been sent to reinforce government ranks, Observatory chief Rami Abdul Rahman told AFP.

Like Hezbollah's militiamen, the Republican Guards have been trained in urban guerrilla warfare, he said.

"The preparations indicate that they are gearing for a major offensive" on neighbourhoods in the north and west of the town still under rebel control, Abdul Rahman said.

A source close to Hezbollah has said 80 percent of Qusayr is now under government control.

"Despite the intense bombardment, the rebels are resisting fiercely," Abdul Rahman said.

He added that Sunni militiamen from Lebanon had joined the battle on the side of the rebels.

"The fighting is becoming more and more sectarian (Shiite versus Sunni) in character," he added.

Syria's regime is dominated by the minority Alawite community, an offshoot of Shiite Islam, while the majority of the population are Sunnis.

Control of Qusayr is essential for the rebels as it is their principal transit point for weapons and fighters from across the border in Lebanon.

It is also strategic for the regime because it is located on the road linking Damascus with the coast, its rear base.

"If Qusayr falls into the hands of the regime, it will be a hard blow for the rebels because routes used to bring in their arms from Lebanon will be closed," said the head of the Britain-based Observatory.

"If Qusayr was not strategic the rebels would not be fighting to the death and the regime and Hezbollah would not have brought in their heavyweights," Abdul Rahman added.

"The fall of Qusayr would also be a blow to the morale of the rebels" who for more than two years have been fighting to topple President Bashar al-Assad's regime.

Iran-backed Hezbollah, a close ally of Assad, sent almost 1,700 fighters to Qusayr more than a week ago to support the regime's assault on the rebel stronghold.

Initially Hezbollah said it wanted only to defend 13 Syrian villages along the border where Lebanese Shiites live, and the Sayyeda Zeinab shrine near Damascus, which is revered by Shiites around the world.

However, its fighters later encircled Qusayr as regime troops prepared for the launch of a withering assault on the town that is home to 25,000 people.