The Syrian Observatory for Human Rights said the Jubar district in the east and nearby Qaboon were heavily shelled, as were the Hajar al-Aswad, Qadam, Assali neighbourhoods in the south.

Fighting between rebels and President Bashar al-Assad's forces erupted at dawn around the Yarmuk camp for Palestinian refugees to the south of the capital, the Britain-based watchdog said.

It added that six rebels were killed in shelling at dawn of the city of Mudamiyeh, just south of the capital, while another rebel bastion, Daraya also in the south, was likewise hit by the regime's artillery.

Clashes erupted at several places along the highway running through the capital's eastern and southern districts, the Observatory said.

The Zamalka neighbourhood, also in the east, meanwhile was bombed by government jets as clashes between loyalists and rebels erupted.

The army on Tuesday launched a major offensive against rebel zones surrounding Damascus as it sought to break the military stalemate in Syria's almost 23-month conflict, which began as a popular uprising against Assad.

Pro-regime newspaper Al-Watan said this week that the army was "determined to crush terrorism around the capital and big cities."

Loyalists troops made ground in the country's north, retaking Karnaz on the strategic Damascus-Aleppo highway on Wednesday after a 16-day onslaught, said Rami Abdel Rahman, head of the Observatory.

The outgunned rebel "fighters withdrew from Karnaz, which they seized in December last year, after heavy fighting and regular forces regained control," he told AFP.

Opposition leader Ahmed Moaz al-Khatib has offered to hold peace talks with Syria's Vice President Faruq al-Sharaa, but Damascus has so far ignored the initiative and has instead intensified attacks on rebel bastions.

The United Nations says more than 60,000 people have been killed in violence across Syria since the outbreak of a revolt in March 2011 that morphed into an insurgency after the army launched a brutal crackdown on dissent.