Heavy clashes between rebels and regular troops erupted in Damascus on Sunday, in the “most intense” fighting in the capital since the start of the anti-regime revolt in Syria 16 months ago, a monitoring group said.

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

“They have never been this intense,” Abdel Rahman told AFP.

He said the fighting was heaviest in the Tadamon, Kfar Sousa, Nahr Aisha and Sidi Qadad neighborhoods.

“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 organizes anti-regime protests in Syria, said plumes of black smoke were Sunday night billowing out of Tadamon and that loud explosions were heard in Nahr Aisha.

The Observatory earlier said violence across Syria on Sunday killed at least 55 people, including 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. 

The Britain-based watchdog estimates that more than 17,000 Syrians have died since the revolt against Bashar al-Assad’s regime began on March 15, 2011.