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 the 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. Six civilians were reported killed in the city.
"(It has) never been this intense," Abdel Rahman told AFP.
"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 late Sunday and that loud explosions had been heard in Nahr Aisha.
The Britain-based Observatory said violence across Syria on Sunday had killed 105 people -- 48 civilians, 16 rebels and 41 soldiers.
The main opposition coalition, the Syrian National Council (SNC), hailed the insurgents fighting army troops in the capital, accusing the regime of having transformed rebel neighbourhoods into "a battlefield."
"The revolution is spreading and has tightened the noose around the regime in zones where it thought it was beyond the anger of the people," SNC spokesman Georges Sabra said in a speech shown on Arab satellite networks.
"We place on the Arab League and the international community the responsibility for any disastrous result from what is going on in Homs and Damascus," he added.
Rights activists say more than 150 people were massacred by Syrian troops backed by pro-regime shabiha militiamen on Thursday in the central village of Treimsa.
If the number is confirmed, this would be one of the bloodiest episodes of the uprising against President Bashar al-Assad, which rights activists say has cost more than 17,000 lives since March 2011.
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 reporters in Damascus.
Makdissi also denied activists' allegations that helicopters and heavy weapons had been used in the assault on Treimsa.
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"This is absolutely not true. Only troop carriers and light weapons were used, the most powerful of weapons being RPGs (rocket-propelled grenades)," he said.
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 spokeswoman 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.
On Monday, UN-Arab League envoy Kofi Annan and UN Secretary General Ban Ki-moon are headed respectively for Russia and China to press the two UN Security Council members to back tougher action against Assad's regime.
The visits by Annan and 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.
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 was 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.