Air raids on rebel-held districts of Syria's battleground second city of Aleppo killed at least 25 civilians including children on Saturday, a monitor said.

The death toll steadily rose throughout the day as bombardment rocked the city, according to the Syrian Observatory for Human Rights.

"Eleven civilians, including four children, were killed by air raids after midnight in the Bab al-Nasr area of Old Aleppo, and seven others were killed in Fardous neighbourhood," the monitor said.

Seven others, including children, were killed in several other rebel-controlled neighbourhoods - among them three in the Salhin district, the Britain-based monitor said.

The Observatory, which relies on a network of sources across Syria for its information, said the air strikes were likely either Russian or regime warplanes.

"At least 20 people are still under the rubble," said Observatory head Rami Abdel Rahman.

Syrian state news agency SANA, for its part, reported that one person was killed and nine others were wounded in rebel rocket fire on government-controlled parts of the city.

An AFP correspondent in eastern Aleppo said helicopters and fighter jets were still circling rebel-held neighbourhoods, adding that barrel bombs - crude, unguided explosive devices - had been dropped on several areas.

A hospital in the Maadi neighbourhood was hit in the bombing, wounding some of the staff and patients inside.

"All kinds of weapons were used to bomb the hospital, from midnight until about 11am. Now it's unusable," Mohammad Kheir, one of its doctors, told AFP.

"There were some injuries among the medical staff but thankfully they are only light wounds."

A crying woman clad in a black robe desperately grasped the leg of a bloodied young man as doctors treated him on the hospital floor.

Twisted metal frames and damaged medical equipment lay strewn across the room, some next to small pools of blood.

Truce routinely violated

The Observatory said rebel fighters shelled government-controlled western areas of Aleppo, but had no immediate word on any casualties.

Aleppo city is divided roughly between government control in the west and rebel control in the east.

It was once Syria's commercial powerhouse but has since been ravaged by the country's five-year war.

A ceasefire brokered by Russia and the United States in February between government forces and moderate rebels does not cover al-Qaeda's affiliate, the Nusra Front, which has a strong presence in many rebel-held areas.

The truce has been routinely violated, particularly in and around Aleppo.

On Friday, US Secretary of State John Kerry and his Russian counterpart Sergei Lavrov said they had agreed on "concrete steps" to salvage the failing ceasefire.

The top diplomats met for a 12-hour marathon meeting, but would not divulge the details of the deal in order to allow the "quiet business" of peacemaking to continue, Kerry said.

Last week, government forces advanced to within firing range of the last remaining supply route into rebel-held areas of Aleppo, prompting food shortages and spiralling prices.

According to the United Nations, nearly 600,000 people are living under siege across Syria, most of them surrounded by government forces although rebel groups also use the brutal tactic.

More than 280,000 people have been killed and millions forced to flee their homes since the Syrian conflict broke out in March 2011.

Earlier this week in an exclusive interview , the leader of the Syrian White Helmets, told Middle East Eye that Russian airstrikes had led to an marked increase in civilian casaulties in rebel held areas.