A missile strike near Syria's biggest city Aleppo killed 26 people and government warplanes pounded Qusayr, a watchdog said, as a regime offensive to retake the town entered its third week.
Regime opponents also suffered a blow when one of the main groups in the National Coalition withdrew from the bloc, denouncing its leadership.
US officials said, meanwhile, that Washington would send a Patriot missile battery and F-16 fighters to Jordan for a drill and may keep them there to counter any threat posed by the Syrian war.
The missile attack on Kfar Hamra came as Assad's forces mounted an assault on the rebel-held countryside surrounding Aleppo in the north.
The Syrian Observatory for Human Rights said the surface-to-surface missile struck around midnight, killing 26 people, including six women and eight children.
"Regime forces... are trying to take the village, and then to break the rebel siege of Nubl and Zahra," two villages north of Aleppo, it added.
The strike comes a few days into an army offensive aimed at advancing on Aazaz, a rebel stronghold north of Aleppo, the most populated city and commercial hub before the war.
Regime forces mounted a fierce onslaught on Qusayr, the strategic town near Lebanon, and also slightly farther north in Dabaa, the site of a disused military airbase partly under insurgent control.
There were numerous dead on both sides, the Observatory said without giving any 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.
Warplanes bombarded Qusayr for the second consecutive day, the Observatory said.
Three missiles also hit the flashpoint town causing serious damage, but it was unknown if there were any casualties.
Raids were also reported on the Al-Hajar al-Aswad district of southern Damascus itself, where pillars of dark smoke rose into the sky.
The Syrian Revolution General Commission announced on Monday it was withdrawing from the National Coalition.
Some Coalition leaders were "more interested in appearing in the media than helping the revolution," said a statement from the opposition faction, a Syria-wide network of activists.
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"A lot of money has been wasted because they used it for their own personal interests while the Syrian people inside the country lack everything," it added.
As fears mount of the conflict spilling over, six people were killed in 24 hours in Lebanon's second city Tripoli during clashes between pro- and anti-Assad Alawite and Sunni residents, a security source said.
Israeli Defence Minister Moshe Yaalon, meanwhile, said Russia would be unable to deliver a shipment of advanced anti-aircraft missiles to Syria before 2014, army radio reported.
Last week, Assad had hinted his government might already have received a shipment of the S-300 missiles.
But Yaalon told an Israeli parliamentary committee: "No deliveries have taken place. If they do take place, it will not be before next year."
In Washington, US Lieutenant Colonel T.G. Taylor said a Patriot missile battery and F-16 fighters being sent to Jordan for a drill may stay there, if requested, "to enhance the defensive posture and capacity" of the country which borders Syria.
On Saturday, UN chief Ban Ki-moon and international aid groups expressed concern about civilians trapped in Qusayr, and for up to 1,500 injured residents still in the town of 25,000 people.
On Sunday, the regime said it would not allow the Red Cross into Qusayr until after the fighting stopped.
US Secretary of State John Kerry on Monday denounced that decision in a fierce attack on Assad.
In Qusayr, he said "...you have an extraordinary number of civilians who are trapped, and he will not allow the Red Cross and humanitarian aid to go in until the military has finished what it intends to do...
"I think the world is seeing the actions of a person who has lost touch with any reality except his own and who is willing to wreak any kind of punishment... on the people of his country simply so that he can maintain power."
At the United Nations, diplomats said that over the weekend Russia had blocked a draft Security Council declaration expressing "grave concern" about the situation in Qusayr.
Moscow, a key ally of Assad's regime, had wanted "wider political discussion" on the issue, they said.
Germany, meanwhile, joined France in saying 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.