A Syrian government assault on the rebel bastion of Qusayr raged into a second day on Monday, with at least 23 members of the Lebanese Shiite group Hezbollah reportedly killed as they fought alongside the army.

The battle began on Sunday, when government troops backed by regime ally Hezbollah stormed the western town, a strategic prize in the country's two-year conflict.

The fighting has raised fears of a "massacre" and cast a shadow over US-Russian efforts to organize a peace conference to discuss a political solution to the war.

The Syrian Observatory for Human Rights said the fighting had left at least 56 rebels dead, six of them on Monday, and four civilians.

"Reliable sources informed the Syrian Observatory for Human Rights that 23 members of Hezbollah's elite forces were killed and more than 70 others wounded in clashes in the town of Qusayr yesterday," said the Britain-based group.

A source close Hezbollah told AFP at least 20 members had been killed in Syria, with around 30 more wounded.

The attack began with an early-morning offensive on Sunday led by warplanes and heavy artillery fire, with ground troops moving in afterwards.

"We struck from several fronts - south, east and northeast," one soldier told state television from the rebel bastion.

A military source told AFP government forces controlled the center of Qusayr and the Syrian flag was flying over the recaptured town hall, though activists denied troops had advanced that far.

Meanwhile, branches of the divided Syrian opposition held talks in Madrid on Monday seeking to harmonize their approach to the country's bloody civil war, their Spanish government hosts said.

The talks included Ahmed Moaz al-Khatib, who resigned last week as leader of the Syrian National Coalition, plus other members of the coalition and "various movements" of the opposition to Syria's President Bashar al-Assad, the Spanish foreign ministry said in a statement.

Also on Monday, an Israeli military spokeswoman said that shots fired from Syria hit the central Israeli-occupied Golan Heights overnight, causing no harm or damage.

According to the spokeswoman, the shots were from small arms, and "most likely were stray bullets, we don't know if it was intentional."

The spokeswoman would not confirm local media reports that the shots hit near an Israeli military patrol. The army had not fired back, she said, and Israel had submitted a complaint to the UN force in the region.

AFP