The Syrian army has deployed tanks on a ring road surrounding Damascus and shelled southern neighbourhoods where rebels operate, the heaviest bombardment in the capital since the army reasserted control last month, residents and activists said.
At least 40 people were killed in the shelling, which was accompanied by attacks from helicopters, and in ensuing ground raids on the Kfar Souseh, Daraya, Qadam and Nahr Aisha neighbourhoods, they said.
"The whole of Damascus is shaking with the sound of shelling," a woman in Kfar Souseh said.
She said the army's artillery was also firing on the capital from the Qasioun and Saraya mountains overlooking Damascus. The assaults in the capital coincide with the departure of the United Nations observer mission, whose members are leaving after failing to secure a ceasefire.
Maaz al-Shami, a member of the Damascus Media Office, a group of young opposition activists monitoring the crackdown in Damascus, said rebels who had left the city during a fierce army campaign last month had started to return.
"They went back to their homes, or disappeared in the green belt surrounding Damascus," Shami said.
"They are back now, and the regime is responding with daily shelling and helicopter bombardment. A war atmosphere in Damascus is setting in."
The renewed attacks followed what activists said was a bloody raid on the Maadamiya neighbourhood on Tuesday.
The Syrian Observatory for Human Rights (SOHR), a London-based anti-government group, said it had documented the names of at least 42 civilians killed in the mixed suburb, which is home to around 200,000 Christians, Alawites and Sunni Muslims.
The SOHR said government troops had targeted a funeral procession, and that dozens of unidentified bodies had been found in a basement as well.
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Heavy shelling and clashes also continued on Tuesday across swathes of Aleppo, the country's largest city, as both the regime and rebels claim they are gaining ground in the key northern battleground.
At least 24 people were reported to have been killed nationwide on Tuesday, among them women and children in Aleppo, as the Syrian government pressed rebel-held areas.
The rebel Free Syrian Army (FSA) claimed that it controlled almost two-thirds of the city, which has been battered by a month of air strikes, shelling and fighting.
"We now control more than 60 per cent of the city of Aleppo, and each day we take control of new districts," Abdel Jabbar al-Okaidi, a colonel with the FSA, said.
He went on to list about 30 districts which he claimed were under FSA control, including about half of the neighbourhood of Salaheddin.
A security source in Damascus rejected the FSA claims, according to the AFP news agency, calling them "completely false".
"The terrorists are not advancing," the Syrian source said.
"It is the army that is making slow progress. Terrorist groups occasionally come out of districts under their control and attack other districts to be able to then claim they have this or that street under their control."
Activists also reported that troops had stormed a town near Damascus, torching homes and shops, while helicopters and war planes strafed several suburbs of the capital, which the regime claimed to have largely recaptured last month.