Opposition fighters have launched a major assault on government-held districts of Syria's long-divided Aleppo, after government forces severed their only remaining supply route into the battleground city.

Rebel groups launched the offensive at dawn on Monday to reopen the strategic Castello Road, their last lifeline into the city, according to the AFP news agency.

The UK-based Syrian Observatory for Human Rights said 19 regime forces were killed on Monday when rebels blew up a tunnel in the Old City.



Observatory chief Rami Abdel Rahman said that despite a heavy assault, rebels had been unable to advance because of the "heavy aerial bombardment the regime is carrying out on the areas where fighting is under way".



Air raids on the rebel-held parts of Aleppo killed 13 civilians on Monday, the Observatory added, most of them in the Bab al-Maqam neighbourhood near the front line.

READ MORE: Why Aleppo matters

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 western area of the city who did not give his second name, said his home in the Syrian quarter was completely destroyed.



"The shells have rained down on the western neighbourhoods since 4:30am," he said.



Mahmud Abu Malak, a spokesman for the Nuruddin al-Zanki rebel group, described fierce fighting.



"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 on Thursday when government forces seized a hilltop within firing range.



The government advance leaves the opposition-held east of the city cut off from the surrounding countryside, 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.

Nationwide truce

The violence comes despite a nationwide truce declared by the government last Wednesday to mark the Muslim holiday of Eid al-Fitr.

The ceasefire has produced little respite in fighting, but was extended for another 72 hours on Saturday.

In nearby Idlib province, 17 people including two children were killed in air strikes, the Observatory said.

The monitor said it was unclear if the strikes were carried out by regime or Russian war planes.

READ MORE: Death of Aleppo

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.

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, most surrounded by government forces, although rebels are also besieging civilians.