SYRIA'S army has blasted rebel strongholds in Damascus with mortars, sparking the "most intense" fighting in the capital since the revolt erupted 16 months ago, a monitoring group says.

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 news 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."

The Local Coordination Committees, which organise anti-regime protests in Syria, said plumes of black smoke were billowing out of Tadamon on 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.

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

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

Originally published as Damascus rocked by intense fighting