There has been an upsurge in fighting in the strategic Syrian town of Qusair, both sides in the conflict say.
The army, which began an offensive to recapture Qusair last Sunday, says it has now launched a three-pronged attack on the rebel-held town.
The rebels say they are under heavy bombardment from Lebanese Hezbollah militants, who back Syria's government.
Meanwhile the main opposition group has said it will attend a peace conference only if the Syrian leader steps down.
The international conference, which Russia and the US are attempting to convene, would take place in Geneva next month.
The opposition said it welcomed the conference but demanded that President Bashar al-Assad leave power first.
A spokesman for the National Coalition, Louay Safi, told the BBC: "We would like to find a political solution. But we don't want to be deceived again by this regime, which has deceived us many times.
Previous efforts to find a political solution to the conflict have foundered on preconditions from the two sides.
Correspondents say Qusair is a strategic conduit for the rebels, through which weapons and fighters can be transported from Lebanon, about 10km (six miles) away.
The town is also important for President Assad because it is located between Damascus and the Alawite heartland of the embattled government on the Mediterranean coast. And it is not far from the motorway linking the capital to the city of Homs.
The week-long fighting in Qusair intensified early on Saturday, when activists reported heavy bombardments, including two ground-to-ground missiles and an air strike as well as artillery and rocket fire.
They say Hezbollah fighters, who are widely reported to have played a major role in the attack on Sunday, were trying to advance again.
"I've never seen a day like this since the battle started," activist Malek Ammar told Reuters news agency from the town. "It's like they're trying to destroy the city house by house."
Syrian state media said the army had launched a three-pronged offensive in the north, centre and south of Qusair, and was making big advances, killing many rebel fighters.
However, official media made no mention of the part played by Hezbollah.
The Lebanese group is also known to have lost a number of fighters in Qusair, prompting Lebanese President Michel Suleiman to warn the Shia militia against getting "bogged down in the sands of discord".
The Syrian Observatory for Human Rights, a UK-based activist group that monitors the conflict, said at least 22 people including 18 rebels had been killed in the latest fighting in Qusair. Dozens had been wounded, it added.
The violence has sparked clashes in Lebanon itself, with fighting between opponents and supporters of Mr Assad in the northern city of Tripoli.
More than 25 people are reported to have been killed there in recent days. The fighting is also reported to have intensified on Saturday.
Hezbollah leader Hassan Nasrallah is due to give his first public comments on the group's involvement in Qusair later on Saturday.