DAMASCUS — The United States voiced fears that Syria's government is planning a new massacre, as regime helicopters fired on rebel stronghold towns and raging violence killed nearly 90 people on Monday.

On the political front, the newly elected leader of Syria's exiled opposition urged 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 87 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.

Such reports prompted Washington to voice concerns that Assad's regime is planning to carry out new atrocities, following the massacre of 55 people last week in Al-Kubeir and at least 108 near Houla on May 25 and 26.

"The United States joins joint special envoy Kofi Annan in expressing deep alarm by reports from inside Syria that the regime may be organising another massacre," US State Department spokeswoman Victoria Nuland said.

Nuland said the tactics showed the Assad regime, cracking down on the most severe threat to his family's four-decade rule, was "increasingly desperate."

"What government voluntarily uses helicopters and fires from them on their own civilians if they're not desperate? What government depends on a bunch of thugs in trucks -- irregulars -- if they're not desperate?" she asked.

But Nuland again ruled out US military intervention.

Russia has strongly opposed the prospect of force to remove Assad, whose family has been a key ally of Moscow since the Cold War.

"The concern has been that putting foreign military forces into this situation -- which is on the verge, as everybody has said, of becoming a civil war -- will turn it into a proxy war," Nuland said.

A spokesman for UN-Arab League envoy Kofi Annan said he was "gravely concerned by the latest reports of violence coming out of Syria and the escalation of fighting by both government and opposition forces."

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," spokesman Ahmad 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."

Women and children trapped

The bloodshed has persisted despite the presence of 300 UN observers charged with monitoring a putative truce launched on April 12.

The mission also expressed concern about the escalation of violence in 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," said the UN Supervision Mission in Syria.

"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 said 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, Abdel Basset 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 grip 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, as happened in a UN-backed transition in Yemen.