Syria's army blasted rebel strongholds in Damascus with mortars on 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 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 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.

Rights activists say more than 150 people were massacred by Syrian troops backed by pro-regime shabiha 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.