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.
- Barrage of shells -
Rebels fired a barrage of at least 300 shells into western Aleppo, killing nine civilians, the Observatory said.
Ahmed, a resident of the government-controlled west of the city, said his home in the Syriaq quarter was completely destroyed.
"The shells have rained down on the western neighbourhoods since 4:30 am," he told AFP.
Residents lifted debris in the Syriaq quarter and helped neighbours gather their belongings so they could leave in search of shelter elsewhere.
Mahmud Abu Malak, a spokesman for the Nureddin al-Zanki rebel group, described fierce fighting.
Signup to our newsletter and follow us on Facebook and Twitter!
"All kinds of heavy artillery and machine-guns are being used in the assault, which is intended to ease the pressure on the Mallah and Handarat fronts," he said, referring to areas near the rebel supply route into Aleppo.
The Castello Road route was effectively severed last Thursday when government forces seized a hilltop within firing range.
The advance leaves the opposition-held east of the city cut off, and raises the prospect of total siege.
On Sunday, at least 29 opposition fighters were killed when rebels launched a fruitless assault to push government forces back from the road and reopen the route.
- Meagre food stocks -
The severing of the Castello Road has already created shortages of food and fuel in the east of Aleppo, with local market stalls sparsely stocked.
"There are very few vegetables today because the Castello Road is closed," said Abu Mohamed, a vendor in the Bustan al-Qasr neighbourhood.
"If we hadn't planted eggplant and zucchini inside the city, we wouldn't have had any vegetables at all," he said, pointing to his meagre stock of vegetables.
Residents also described searching in vain for fuel, whether for vehicles or home use.
The UN says nearly 600,000 Syrians live in besieged areas of the country, most surrounded by government forces, although rebels also use the tactic.
The latest violence comes despite an extension until early Tuesday of a nationwide truce declared last week by the government to mark the Muslim holiday of Eid al-Fitr.
In northwest Idlib province, 17 people including two children were killed in air strikes which the Observatory said were carried out by either regime or allied Russian warplanes.
US Secretary of State John Kerry will travel to Moscow this week to seek common ground on how to deal with the ongoing bloodshed, it was announced Monday.
The current truce does not cover jihadists from the Islamic State group or the Al-Qaeda-linked Al-Nusra Front.
Also Monday, IS jihadists entered the historic city of Palmyra in central Syria for the first time since being defeated there by regime forces in March.
But they were killed and the attack was now over, according to the Observatory.
IS fighters also claimed a suicide attack on a rival rebel group in the town of Dumeir, east of Damascus, on Sunday that killed 16 people.