ALEPPO - Clashes and air strikes shook the Syrian city of Aleppo on Sunday, as heavy fighting resumed after the end of a three-day ceasefire declared by government ally Russia.

The unilateral ceasefire ended without any evacuations by the UN, which had hoped to bring wounded civilians out of the rebel-held east and deliver aid after weeks of government bombardment and a three-month siege.

An AFP correspondent in the east of the city reported fresh air strikes on rebel-held neighbourhoods and the sound of fighting on Sunday afternoon. The Syrian Observatory for Human Rights said strikes and artillery fire hit eastern districts on Sunday after heavy clashes overnight along the front line that divides the city's government-held west from the east.

Rebel shelling also hit two western neighbourhoods on Sunday, the Britain-based monitor said.

The Observatory late on Saturday had reported the first air strikes since Moscow announced a temporary halt in the Syrian army's Russian-backed offensive to recapture the east of the city.

It said at least three people were wounded in artillery fire on the east of the city, while rebels fired a barrage of rockets and mortar shells on a government-held neighbourhood.

Government forces and allied fighters, meanwhile, had advanced on the southern outskirts of Aleppo, the Observatory said on Sunday, seizing territory overlooking the rebel-held areas of Rashideen and Khan Tuman.

The fighting killed at least 20 fighters, mostly from the former Al-Qaeda affiliate, the Fateh al-Sham Front, the monitor said. The ceasefire was intended to allow civilians and rebels to leave the east.

The army had opened eight evacuation corridors, but only a handful of civilians were reported to have crossed through a single passage. Russian officials and Syrian state media accused rebels of preventing people from leaving and using civilians as "human shields".

Nearly 500 people have been killed and more then 2,000 wounded since the Syrian army launched a September 22 operation to recapture eastern Aleppo.

The United Nations had hoped to use the "humanitarian pause" to evacuate seriously wounded people and possibly deliver aid. But a UN official said on Saturday the requisite security guarantees had once again not been received. "You have various parties to the conflict and those with influence and they all have to be on the same page on this and they are not," said David Swanson, a spokesman for the UN humanitarian office.

No aid has entered Aleppo since July 7 and UN chief Ban Ki-moon has warned food rations will run out by the end of the month.

The UN had asked Moscow to consider extending the pause until Monday evening, but there was no indication from Russia that it would.

Russia is a key ally of Syria's government and began a military intervention in support of President Bashar al-Assad in September last year.

Kremlin spokesman Dmitry Peskov said in an interview aired on Saturday that the intervention was meant to "liberate" Syria and keep Assad in power.

"Either Assad is in Damascus, or Al-Nusra is," he said, referring to Fateh al-Sham's name before it broke with Al-Qaeda. "There is no third option here."

Elsewhere, at least two people were wounded on Sunday when a bomb strapped to a motorbike exploded in the northeastern city of Hasakeh, the Observatory said.

The blast in a Kurdish-controlled district is one of a series to have hit the city, most claimed by the Islamic State group.

More than 300,000 people have been killed in Syria and more than half of the country's population displaced since the conflict began in March 2011 with anti-government protests.

French Foreign Minister Jean-Marc Ayrault on Sunday urged the international community to "do everything" to end the "massacre" in Aleppo and resume efforts to reach a political agreement.

"We cannot come to a negotiation under the bombs," he said.

On Saturday, the White House condemned the Syrian regime's "defiance" of international law after UN experts said the army was responsible for a chemical weapons attack in March 2015.

It was the third chemical weapons attack the UN panel has attributed to government forces.

"We condemn in the strongest possible terms the Assad regime's defiance of the longstanding global norm against chemical weapons use," said US National Security Council spokesman Ned Price.