Rebel fighters launched a major assault on government-held districts of Syria's long-divided Aleppo on Monday, after the regime severed their only remaining supply route into the battleground city.
Once Syria's economic powerhouse, Aleppo has been ravaged by the war that began with anti-government protests in 2011 and which has since killed more than 280,000 people.
Rebels launched the offensive at dawn to reopen the Castello Road, their last lifeline into the northern city, an AFP correspondent said.
The Britain-based Syrian Observatory for Human Rights said 19 regime forces were killed Monday when rebels blew up a tunnel in the Old City.
Observatory chief Rami Abdel Rahman said: "The opposition has not advanced because of the heavy aerial bombardment the regime is carrying out on the areas where fighting is underway."
Air raids on the rebel-held parts of Aleppo killed 13 civilians Monday, the Observatory said, most of them in the Bab al-Maqam neighbourhood near the front line.
An AFP journalist saw rescue workers help a stocky, shirtless man out of the rubble in Bab al-Maqam, his face and thick beard caked in dust.
The unidentified man entered a field hospital and embraced the limp bodies of two young boys. "He was martyred. He's gone," he said crying over the body of one them.
Several rounds of UN-brokered talks to end the conflict have failed, but the UN's special envoy Staffan de Mistura said Monday that a "crucial moment" had been reached in efforts to secure a political settlement.
But despite the diplomatic flurry, fighting has intensified in Aleppo, divided between government forces in the west and rebels in the east since mid-2012.