Syrian troops backed by fighters from Lebanon's Hezbollah attacked and entered the strategic rebel stronghold of Qusayr on Sunday, a day after President Bashar al-Assad insisted he would not quit.

The advance came as opponents warned the Assad regime's "barbaric and destructive" assault on Qusayr could render "meaningless" US-Russian attempts to organize a conference on ending two years of bloodshed in the country.

The Arab League called an emergency meeting for Thursday, ahead of the conference, following demands from the opposition Syrian National Council for it to meet and "stop the massacre in Qusayr."

The Syrian regime's assault on Qusayr - located on the key road between Damascus and the Lebanese border - followed heavy early morning bombardment of the town by its artillery and warplanes.

"The assault on Qusayr has started. There is fierce fighting between rebels and the army around the entrances to the town," said Rami Abdel Rahman of the Syrian Observatory for Human Rights.

"Soldiers and tanks are trying to advance into the town, the rebel forces are attempting to push them back," he said.

Troops were entering from the south, and fighters from the Lebanese militant group Hezbollah, a key ally of the Syrian regime, were "playing a central role," he added.

State television said the army had "tightened the noose on the terrorists, attacking on different fronts and destroying positions of their leaders in the south of the town."

At least 30 people, including 16 rebel fighters, were killed in the fighting, said the Observatory, a Britain-based group that relies on various sources on the ground.

"The Syrian army controls Qusayr's main square in the center of the city, and the surrounding buildings, including the municipality building," a military source told AFP.

The regime has made recapturing Qusayr and the surrounding district of Homs province a key objective, and fierce fighting has raged in the vicinity for months.

Israeli Prime Minister Benjamin Netanyahu held out the prospect on Sunday of further Israeli strikes inside Syria, pledging to act to prevent advanced weapons from reaching Hezbollah and other militant groups.

Although Israel has not publicly taken sides in Syria's civil war, Western and Israeli sources say it has launched air strikes in Syria to destroy weapons it believed were destined for Hezbollah.

In public remarks at the weekly meeting of his cabinet, Netanyahu made no direct mention of those attacks, but made clear Israel was prepared to take action in the future and said it was "preparing for every scenario" in the Syrian conflict.

Israel had set a policy "to prevent, as much as possible, the leakage of advanced weapons to Hezbollah and terror elements," he said.