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 hard.
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 barrelled into the sky.
The Syrian Revolution General Commission said 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 opposition faction, a Syria-wide network of activists, said some Coalition leaders were "more interested in appearing in the media than helping the revolution".
As fears mount of the conflict spilling over, six people were killed in 24 hours in Lebanon's second city Tripoli as pro- and anti-Assad Alawite and Sunni residents clashed, a security source said.
Israeli Defence Minister Moshe Yaalon, meanwhile, said Russia will be unable to deliver a shipment of advanced anti-aircraft missiles to Syria before 2014.
Last week, Assad had hinted his government may already have received a shipment of the S-300 missiles, but Yaalon rebutted this.
"No deliveries have taken place. If they do take place, it will not be before next year," he told Israeli army radio.
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.
Monday's fighting in Syria came a day after the regime said it would allow the Red Cross into Qusayr only when the fighting 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, a key ally of Assad's regime, was demanding "wider political discussion" on the issue.
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".
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.