Syria's army has extended a nationwide truce for another three days, but continued Tuesday to press its campaign against rebels in the battered northern city of Aleppo. 

In a statement late Monday, the armed forces said it would "extend the freeze on fighting on all Syrian territory for 72 hours beginning at 00:01 on July 12."

It was the second extension to the temporary truce, first announced last week to mark Eid al-Fitr, which is the end of the Islamic holy month of Ramadan.

But the ceasefire has produced little respite in fighting, with ongoing strikes by the regime and its Russian ally reported around the city of Aleppo, the Syrian Observatory for Human Rights monitor said.

"Russian and regime warplanes are shelling the northern edges of the city and the regime seized several buildings in Leramun," said Observatory head Rami Abdel Rahman. 

Rebels were using Leramun, an industrial district on the northwestern edge of Aleppo, and the nearby Bani Zeid neighbourhood to fire rockets and artillery onto regime-held parts of the city, he said.

"The immediate goal is to push opposition fighters out of the city to stop the shelling," he told AFP. 

In recent days, rebels have fired a barrage of missiles into the government-held western side of Aleppo, killing dozens of civilians.

Rebel forces in the eastern part of Aleppo have also waged battles with regime troops along the frontline that divides the city between government and opposition control.

Abdel Rahman said there was little evidence of the truce taking effect anywhere in the country.

"There are clashes everywhere -- Homs, Hama, Latakia, Aleppo. Only Daraa is quiet," he said, referring to the southern province. 

Aleppo city, once Syria's economic powerhouse, has been ravaged by the conflict that began in March 2011 and has killed more than 280,000 people.

On Thursday, government forces effectively severed the last supply route into the rebel-held east, the Castello Road, when they took a hilltop within firing range. 

The Britain-based Observatory said air strikes hit the Castello Road and the surrounding area on Tuesday.

Rebels launched a counter-attack to reopen the road but have made no progress. 

With the route effectively shut, there are concerns that the roughly 200,000 residents of rebel-controlled neighbourhoods, according to the Observatory, could face a long siege.

Civilians have already reported shortages of food and fuel in the east of Aleppo, with local market stalls sparsely stocked and prices rising.

The United Nations says nearly 600,000 Syrians live in besieged areas of the country, most surrounded by government forces, although rebels also use the tactic.

On Tuesday, the UN said it was "deeply concerned" about the situation in and around Aleppo.

UN spokeswoman Alessandra Vellucci said most people in eastern Aleppo "rely heavily on humanitarian assistance" and that access to that side of the city was now "virtually impossible."

Source: AFP