Regime forces pounded Rastan, home to a large number of rebel fighters in central Syria, at an average rate of "one shell a minute," the Syrian Observatory for Human Rights said.
Most of Rastan's residents have fled after months of fighting, but regime forces have been unable to regain control of the town. On May 14, 23 regular troops were killed in fighting during a failed assault.
Six people were also killed across Syria, the Observatory said, including three in a blast on the international airport road in the suburbs of Damascus, in the latest sign of increasing violence in the Syrian capital.
Other blasts were heard in several Syrian provinces on Tuesday night, including in Harasta and Douma, two suburbs of Damascus that have seen fierce fighting between regime and rebel forces in past days.
A total of 26 people, including 14 civilians and 12 soldiers, were killed in violence on Tuesday, the Observatory said, adding that Syrian forces carried out a spate of raids in Damascus overnight.
Fighting has grown increasingly violent around Damascus, Aleppo and northwest Idlib, despite the presence of some 270 UN truce monitors on the ground.
The raging violence in Syria took a broader turn in the region after Lebanon's state news agency said Syrian rebels kidnapped 13 Lebanese Shias as they were headed home by bus from a pilgrimage in Iran.
Lebanese Foreign Minister Adnan Mansur said Wednesday that the pilgrims were abducted by "a splinter group of the armed Syrian opposition", but added he expected their release "within hours."
"According to information provided by an Arab country those kidnapped will be free within hours," Mansur told al Jadeed, a private satellite television station.
The Free Syrian Army, which wants to overthrow President Bashar al Assad's regime, strongly denied abducting the Lebanese pilgrims.
"The FSA is not at all responsible for the operation," Mustafa al Sheikh, a high-ranking FSA officer, told AFP by telephone from Istanbul.
"This is an attempt to distort the image of the FSA. The FSA does not believe in this methodology," said Sheikh, the head of the group's military council.
Sheikh said the Syrian regime "treats the FSA as a scapegoat. We condemn this abduction, which does not represent the values of the (Syrian) revolution."
The kidnapping "is no doubt the work of the regime, which wants to sow chaos in the region," Sheikh added.
Syria's main opposition coalition also issued a call for the prompt release of the group, adding that it too believed the Syrian regime could be involved in the kidnapping.
The Syrian National Council "does not think it is impossible that the regime is involved in this operation," in order to sow "disorder" in neighbouring Lebanon, the group said.
But Lebanese women pilgrims who arrived in Beirut early on Wednesday said the kidnappers presented themselves as belonging to the FSA. "They terrorised us," said one of them.
The case has triggered fears of sectarian tensions in Lebanon over the revolt in neighbouring Syria.
The news prompted their families and thousands of supporters to pour out into the streets of Beirut's mainly Shia southern suburbs on Tuesday night to demand their release.
Protesters blocked several roads, including the old airport road, with burning tyres and garbage bins.
Lebanese Shia leader Hassan Nasrallah, a strong ally of the embattled regime in Damascus, appealed for calm and said his Shia party was doing its utmost to ensure the safe release of the men.
"I call on everyone to show restraint," Nasrallah said in a televised speech.
Nasrallah said contacts were underway with Syrian authorities and other countries in the region for a quick resolution. "We will work day and night until those beloved are back with us," he vowed.
Lebanon's northern Akkar region has also seen protests this week after the weekend killings of two clerics at an army checkpoint in Akkar, a mainly Sunni region whose inhabitants are hostile to Assad.
The killings ignited street battles in the capital Beirut on Monday that left two people dead and 18 wounded.
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