Syrian warplanes pounded the embattled town of Qusayr on Monday as a regime offensive backed by fighters from Lebanon's Hezbollah to retake the town from rebels entered its third week.
The onslaught came as the opposition was rocked by the withdrawal of one of the main groups from the main Syrian National Council, amid accusations the coalition's leaders were misusing funds and acting in their own interests.
The Syrian Observatory for Human Rights reported fierce overnight fighting both in the strategic town near the border with Lebanon and slightly farther north of Qusayr in Dabaa.
Dabaa, the site of a disused military airbase that had been seized by rebels battling President Bashar al-Assad's regime, is still partly under insurgent control.
The Britain-based Observatory, which relies on a wide network of activists, medics and others on the ground for its information, said there were numerous dead on both sides, but gave no details.
An estimated 94,000 people have been killed in Syria since a peaceful protest movement that began in March 2011 quickly became an armed revolt when the regime cracked down hard.
Warplanes bombarded Qusayr for the second consecutive day, the Observatory said on Monday.
Three missiles, believed to be surface-to-surface, also hit areas of the flashpoint town causing serious damage, the Observatory reported, but it was not known whether there were casualties.
The watchdog also reported air strikes on the Al-Hajar al-Aswad district of southern Damascus itself, where pillars of thick dark smoke barrelled into the sky.
It said clashes between soldiers and rebels were ongoing in the Abasiyeen area outside the capital's Jubar neighbourhood.
Again, the rights group was unable to give a casualty figure.
The Syrian Revolution General Commission said in a statement it was withdrawing from the Coalition because it was "taking initiatives far removed from the true revolution and cannot represent the revolution in an authentic way".
The SRGC, which is made up of a network of activists across Syria, said some Coalition leaders were "more interested in appearing in the media than helping the revolution".
As fears mount of the conflict spilling over mount, four people were killed and 21 wounded in Lebanon's second city Tripoli as pro- and anti-Assad Alawite and Sunni residents clashed, a security source said.
The renewed clashes in Tripoli followed a brief lull in the violence between the two sides in the opposing Jabal Mohsen and Bab el-Tabbaneh areas, after a flare-up last month left 31 people dead.
In the southern Lebanese city of Saida, a Sunni imam allied to Hezbollah said he had been fired on, and another Sunni cleric aligned with Hezbollah said gunmen shot up his car in the Bekaa.
Israeli Defence Minister Moshe Yaalon, meanwhile, said Russia cannot deliver a shipment of advanced anti-aircraft missiles to Syria before 2014.
Assad had hinted in a television interview on Thursday that he may already have received a shipment of S-300 missiles, but Yaalon rebutted this.
"We are following this matter with concern, but no deliveries have taken place. If they do take place, it will not be before next year," he told Israel's parliamentary foreign affairs and defence committee, in remarks reported by army radio.
Monday's fighting in Syria came a day after the regime said it would allow the Red Cross into Qusayr only when the fighting there stopped.
At the United Nations, diplomats said Russia blocked a draft Security Council declaration expressing "grave concern" about the situation in Qusayr.
They said Moscow was demanding "wider political discussion" on the issue.
Russia is a key ally of the Assad's regime, which is also backed by Shiite Iran.
On Saturday, UN chief Ban Ki-moon and international aid groups expressed concern about civilians trapped in Qusayr, and for between 1,000-1,500 injured residents still in the town of 25,000 people.
But on Sunday, Syrian Foreign Minister Walid Muallem expressed surprise "given that no one expressed this concern when terrorists took control of the city and the surrounding area".
France, meanwhile, said that the proposed "Geneva 2" peace conference on ending the bloodshed in Syria could be delayed until next month.
The international community has pinned its hopes for resolving the conflict peacefully on the US-Russian initiative that had been mooted for June in Geneva.