Fierce battles raged on the edges of Damascus Tuesday as the army pressed a major assault to crush rebels around the capital, a monitoring group and activists said.
And in the contested city of Aleppo in the country's north, rebels attempted to advance into western regime-held districts, sparking clashes with government forces, the Syrian Observatory for Human Rights said.
"The army is trying to take over Qaboon, Barzeh, Jubar, Al-Hajar Al-Aswad and Yarmuk," said Observatory director Rami Abdel Rahman, referring to neighbourhoods in the northern, eastern and southern outskirts of the capital.
"The army doesn't have the capacity to take over these neighbourhoods, and the rebels are fighting back. But the humanitarian situation there is catastrophic," Abdel Rahman told AFP.
An activist in Qaboon said the army's offensive on the capital's northeastern district entered its sixth day on Tuesday.
"The district is the only entrance (from the east/northeast) into the capital... Regime troops fear the (rebel) Free Syrian Army will use Qaboon to enter into Damascus," said the activist, who identified himself as Anas.
Speaking to AFP via the Internet, Anas described a critical lack of medical and food supplies in his neighbourhood.
"The humanitarian situation is so bad it would make anyone cry," he said.
Activists meanwhile said the army shelled other rebel-held areas around Damascus in its bid to drive rebel forces out of the capital.
Signup to our newsletter and follow us on Facebook and Twitter!
The Local Coordination Committees, a grassroots network of activists, reported shelling on Yarmuk Palestinian camp in southern Damascus.
And the Syrian Revolution General Commission reported that army tanks pounded rebel stronghold Daraya southwest of the capital, keeping up a months-long campaign to crush the insurgency there.
In northern Syria, meanwhile, clashes in western Aleppo raged on, days after rebels launched an offensive on regime-held neighbourhoods there.
"The rebels and the army are engaged in tit-for-tat operations ... in Rashidin and Ashrafiyeh" in the west of Aleppo, said the Observatory's Abdel Rahman.
The rebels' first major advance on Aleppo took place nearly a year ago. Though they took control of a large number of neighbourhoods during the assault, the city has been at a near-standstill for many months, with neither side making significant advances.
Meanwhile in central Homs, violence raged in Tel Kalakh and Qaritin villages, as Assad loyalists pressed an assault on the countryside near the city dubbed by activists "the capital of the revolution".
In Qaritin, rebels battled the army as it tried to storm the town, said the Observatory. Meanwhile, troops summarily executed a detainee from Tel Kalakh as they pressed an attempt to take over the rebel town, it added.
More than 93,000 people have been killed in Syria's 27-month war, says the UN.
Millions have been forced by the violence to flee their homes.