Syrian forces tried to storm the rebel bastion of Rastan under cover of gunfire and shelling, reports said, as Damascus admitted sanctions were biting and the head of the main opposition bloc resigned.
The rebel Free Syrian Army (FSA) meanwhile denied it was behind the abduction of Lebanese Shiite pilgrims, an incident stoking tensions in neighbouring Lebanon which is divided between pro- and anti-Damascus camps.
There was no word on any Rastan casualties, but the London-based Syrian Observatory for Human Rights said 15 people were killed nationwide, including seven troops shot dead at Qalamun in Damascus province as they tried to defect.
As the violence continued to rage, Russia, one of Syria's few allies, offered to host direct talks between the Damascus regime and rebel representatives, Deputy Foreign Minister Mikhail Bogdanov said.
Soldiers were trying to overrun Rastan for the second time in 10 days, with shells crashing into the town at the rate of "one a minute" at one stage, according to the Britain-based Observatory.
An activist said FSA fighters were defending Rastan's entrances but that "regime forces are being strengthened with new deployments," including from the elite Republican Guard.
"Electricity has been cut off in Rastan, and water tanks have been shelled," Abu Rawan told AFP. "There is also a severe lack of food because the market is closed and we can't bring food in from nearby villages."
Hours later, the activist said the army assault eased when a team of UN observers entered Rastan.
On May 14, 23 soldiers were killed in a failed assault on Rastan, a town straddling the main highway linking Damascus to the north and where rebels regrouped from the battered city of Homs.
Elsewhere, troops fired on protesters in second city Aleppo in the north as about 1,500 people rallied against the regime, triggering armed clashes, said the Observatory.
Activists said lawyers and sympathisers with the revolt staged a sit-in at Aleppo's judicial complex to demand the release of political prisoners, and to pay tribute to four students killed at a May 3 rally in the city.
The uprising against President Bashar al-Assad broke out with peaceful democracy protests in March 2011, prompting a fierce crackdown that spawned an increasingly militant response.
More than 12,600 people have been killed, nearly 1,500 of them since a UN-backed truce took effect April 12, according to the Observatory.
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The bloodshed has persisted despite the ceasefire brokered by UN and Arab League envoy Kofi Annan, as well as previous diplomatic measures including several rounds of sanctions against Assad's regime.
The violence took a broader turn in the region as Lebanon's state news agency said Syrian rebels kidnapped 13 Shiites as they were headed home to Lebanon by bus from a pilgrimage in Iran.
Lebanese Foreign Minister Adnan Mansur said the pilgrims were abducted by "a splinter group of the armed Syrian opposition", but that he expected their release "within hours."
The FSA denied involvement.
The opposition Syrian National Council, whose leader Burhan Ghalioun formally resigned from his post Wednesday, also issued a call for the pilgrims' release, adding it too believed the regime could be involved in the kidnapping.
Ghalioun had announced plans to resign on May 17 to avert divisions within the opposition bloc, after activists on the ground accused him of monopolising power.
Oil Minister Sufian Allaw admitted on Wednesday that punitive measures imposed by the West have cost Syria almost $4.0 billion (3.2 billion euros) and caused shortages in fuel products.
"The oil sector has lost almost $4 billion because of the unjust European and US sanctions, blocking exports and imports of oil and oil derivatives," he told a news conference.
The minister acknowledged "new difficulties" in meeting Syria's energy needs, especially for domestic gas, blaming measures taken by the United States and the European Union.
Speculation was also rife Wednesday among anti-regime activists over the alleged "killing" of Assad's brother-in-law Assef Shawkat, who is also Syria's deputy defence minister.
Shawkat, former head of military intelligence, was poisoned, according to anti-regime activists. The authorities in Damascus have not responded publicly to the claim.
Amnesty International said Thursday the Syrian government may have committed crimes against humanity by using "lethal" force and "torturing" detainees in its crackdown.
And Google announced that its mapping software and other products would be available in Syria after getting export approval by the US government, citing the right to "free expression."
Over the past 14 months of unrest, pro-democracy protesters have used social networking sites to whip up support and the government has also resorted to the Internet to strike back.