Syria's army blasted revolution 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 forces 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 forces 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 neighborhoods, 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 neighborhoods but so far they have not succeeded," he added.

The Local Coordination Committees, which organize 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 main opposition coalition, the Syrian National Council (SNC), hailed the revolutionaries fighting army troops in the capital, accusing the regime of having transformed neighborhoods 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.

The UN Supervision Mission in Syria said a team of "specialized 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."

PHOTO CAPTION

In this citizen journalism image taken on Friday, July 13, 2012 and provided by Edlib News Network ENN, Free Syrian Army soldiers run for cover in Idlib province, northern Syria.

AFP