Syria's army blasted rebel strongholds in Damascus with mortars Sunday, sparking the 'most intense' fighting in the capital since the revolt erupted 16 months ago, a monitoring group said.

The army's offensive, aimed at driving rebels of the Free Syrian Army (FSA) out of Damascus, was launched soon after Syria's foreign ministry held a press conference to deny its troops had carried out a massacre in Treimsa village.

"The regular army fired mortar rounds into several suburbs" where FSA rebels are entrenched, said Rami Abdel Rahman, head of the Syrian Observatory for Human Rights.

The fighting was heaviest in the Tadamon, Kfar Sousa, Nahr Aisha and Sidi Qadad neighbourhoods, he said.

"(It has) never been this intense," Abdel Rahman said.

"The security forces are attempting to take control of these neighbourhoods but so far they have not succeeded," he added.

The Local Coordination Committees, which organise anti-regime protests in Syria, said plumes of black smoke were billowing out of Tadamon Sunday night and that loud explosions had been heard in Nahr Aisha.

The Observatory earlier said violence across Syria on Sunday had killed at least 55 people. They included a girl who died along with three other people when the army rained shells on the town of Rastan, a rebel stronghold in the central province of Homs.

Rights activists say more than 150 people were massacred by Syrian troops backed by pro-regime shabiha militiamen on Thursday in the village of Treimsa in central Syria.

If the number is confirmed, this would be one of the bloodiest episodes of the uprising.

But foreign ministry spokesman Jihad Makdissi vehemently denied both the allegations of a massacre and the number of people reportedly killed.

"What happened was not a massacre ... It was a clash between regular forces and armed groups who do not believe in a peaceful solution," Makdissi told the reporters in Damascus.

"What happened was not an attack by the army on innocent civilians," said Makdissi. He also denied activists' allegations that helicopters and heavy weapons had been used in Thursday's assault on Treimsa.

"This is absolutely not true. Only troop carriers and lights weapons were used, the most powerful of weapons being RPGs (rocket-propelled grenades)," he said.

'Only five buildings targeted'

Makdissi said "only five buildings, where there were very sophisticated weapons were targeted".

And dismissing allegations of more than 150 killed, he said that "37 armed men were killed and two civilians only", citing an unidentified source who claimed to have buried them.

The UN Supervision Mission in Syria said a team of "specialised civilian and military experts" had visited Treimsa on Sunday to continue their investigation into the reports of the mass killing.

"The integrated patrol ... observed over 50 houses that were burned and/or destroyed. Pools of blood and brain matter were observed in a number of homes," UNSMIS spokesman Sausan Ghosheh said in a statement.

"On the basis of some of the destruction observed in the town and the witness accounts, the attack appears targeted at army defectors and activists," she added.

"The number of casualties is still unclear."

The Treimsa incident has galvanised international diplomatic efforts over the crisis.

Kofi Annan and Ban Ki-moon are headed respectively for Russia and China on Monday to press the two UN Security Council members to back tougher action against President Bashar al-Assad's regime.

The visits by Annan, the UN-Arab League envoy, and UN leader Ban, come at a crucial new stage in the conflict.

The Security Council has until Friday to renew the UN mission in Syria but is divided over Western calls to add sanctions.

"So divided that maybe Annan and Ban now have the most influence over Russia and China to get anything done," said one senior UN council diplomat, speaking on condition of anonymity.

The international community has reacted with outrage to the latest killings, which have added urgency to deadlocked UN Security Council negotiations on a Syria resolution.

A draft statement which said the Syria government was in "violation" of its international commitments was circulated among the 15 Council nations on Friday, diplomats said.

Russia's envoys said they could not agree. Russia has led the resistance and Annan is to meet President Vladimir Putin and Foreign Minister Sergei Lavrov during two days of talks in Moscow, said his spokesman Ahmad Fawzi.

China has supported Russia's position and Ban heads for Beijing on Monday, officially for a China-Africa summit.

But Syria will top his agenda when he meets President Hu Jintao and other senior leaders, said a UN official.