Regime helicopters fired on rebel stronghold towns on Monday as violence across Syria killed dozens of people and envoy Kofi Annan said he was gravely concerned by the intensifying clashes.
Abdel Basset Sayda, the newly elected leader of Syria's exiled coalition, called on President Bashar al-Assad to hand over power to his deputy Faruq al-Shara, in line with a plan based on a UN-backed power transfer in Yemen.
The Syrian Observatory for Human Rights said nationwide violence cost the lives of at least 74 people on Monday, adding to its overall toll of more than 14,100 killed since an anti-regime revolt erupted in March 2011.
Government helicopter gunships strafed rebel positions in Al-Heffa, as well as the opposition stronghold of Rastan, in the central Syrian province of Homs.
An activist broke down in tears as she told AFP via Skype that tanks were parked on the edge of Al-Heffa, a town of 30,000 people set in rugged countryside in the northwest near the Turkish border.
"They have never come this close before," Sem Nassar said, adding: "There's only one doctor working to treat the wounded in the town," and that most residents had fled.
UN and -Arab League peace broker Annan was "gravely concerned by the latest reports of violence coming out of Syria and the escalation of fighting by both government and opposition forces," his spokesman Ahmad Fawzi said.
The former UN secretary general, who drew up a faltering plan aimed at ending the bloodshed, was referring to shelling in Homs and the reported use of mortars, helicopters and tanks in Al-Heffa.
"There are indications that a large number of civilians are trapped in these towns," Fawzi said.
"The Joint Special Envoy demands that the parties take all steps to ensure that civilians are not harmed, and further demands that entry of the UN Military Observers be allowed to the town of Al-Heffa immediately."
The bloodshed has persisted despite the presence of 300 UN observers charged with monitoring a putative truce launched on April 12.
The UN observer mission in Syria also expressed concern about the escalation of violence in the central city of Homs, and said it was trying to negotiate the evacuation of civilians.
"The impact from heavy artillery shelling and machinegun firing was heard and seen over the neighbourhood of Khaldiyeh in the city centre," the UN Supervision Mission in Syria said in a statement.
"UNSMIS has also received reports of a large number of civilians, including women and children trapped inside the town and are trying to mediate their evacuation."
It added that reports of a large number of casualties could not yet be confirmed.
Activists said Khaldiyeh and two other neighbourhoods were under siege and Red Crescent teams were denied access.
UN observers also reported heavy fighting in Rastan and Talbiseh, north of Homs "with artillery and mortar shelling, as well as firing from helicopters, machine guns and smaller arms," the UN statement said.
Observers reported that in Talbiseh, the rebel Free Syrian Army (FSA) captured Syrian soldiers.
With Annan's peace plan faltering, world powers are divided on how to stop the crisis. The West has called for tougher sanctions and for Assad's departure, while Russia and China reject any foreign interference.
Russian Foreign Minister Sergei Lavrov will visit Iran on Wednesday to discuss an international conference on Syria that Moscow wants to host, among other issues.
France said it will talk to Russia about the idea, as it urged the new head of the Syrian National Council, Sayda, to unite the opposition.
Sayda, elected leader of the exiled coalition on Sunday, called on Assad to transfer power to Shara, the Anatolia news agency reported.
"Assad should leave office to his vice president," Sayda told the Turkish news agency.
Sayda said Syria's leadership was losing its control of the country day by day, claiming that "the regime is able to maintain its control only over a few streets (in Damascus)."
Russia, Assad's main ally, is under growing pressure to back his departure as a first step in a peace accord that would see his inner circle assume command in the interim, based on a UN-backed transition in Yemen.
The FSA has called for a campaign of civil disobedience and urged officers and troops in Assad's military to defect to the opposition.
Meanwhile, a blast damaged a gas pipeline linking eastern Deir Ezzor and Homs on Monday, causing the leakage of 400,000 cubic metres of gas, state news agency SANA said, blaming "terrorists."
The authorities accuse foreign-backed "terrorist groups" -- its term for rebel forces -- of attacking infrastructure but the opposition says the regime is involved in the attacks.
In neighbouring Lebanon, political rivals agreed the country must not become a base for smuggling arms and insurgents into Syria while rejecting the idea of establishing a buffer zone along their shared border.