Troops backed by helicopters pushed an offensive against rebels in Syria's commercial capital Aleppo into a second straight day on Sunday, sparking fierce fighting and sending civilians fleeing.
The opposition Syrian National Council (SNC) accused the government of preparing to carry out "massacres" in the northern city and pleaded for heavy weapons to enable rebel fighters to meet the onslaught.
Syrian foreign minister Walid al-Muallem, on a surprise visit to key ally Iran, said the rebels "will definitely be defeated" in Aleppo, even as a Free Syrian Army (FSA) commander boasted the city would become a "graveyard" for the army's tanks.
Peace envoy Kofi Annan urged both sides to hold back, saying that only a political solution could end a conflict that human rights monitors say has killed more than 20,000 people since the uprising erupted in March 2011.
The Britain-based Syrian Observatory for Human Rights said Sunday's fighting was focused around the southwestern neighbourhood of Salaheddin, where rebels repulsed a ground assault on Saturday.
"There are clashes on the edges of... Salaheddin," Observatory head Rami Abdel Rahman told AFP.
"Regime forces are also using helicopters to pound the district. Fighting is also happening in the central neighbourhood of Bab al-Hadid."
Abdel Rahman described the situation in Aleppo as "a full-scale street war," with fighting in the Sukari, Fardoss and Jisr al-Hajj neighbourhoods. Clashes were also reported in Zahraa and Arkub and at the Al-Hindrat Palestinian refugee camp, the Observatory reported.
Rebel forces broke into a juvenile detention centre, said Abdel Rahman, "in order to set the prisoners free."
Families displaced by the fighting in Aleppo were having difficulty finding refuge "because nowhere is safe any more," he added.
After massing for two days, troops backed by tanks and helicopters on Saturday launched a ground assault on Salaheddin, where rebels concentrated their forces when they seized much of Aleppo on July 20.
Both sides claimed to have made advances, but an AFP correspondent reported rebels had largely repulsed the army when it launched its first onslaught on Saturday.