At least 32 civilians, including five children, were killed Thursday in Syrian regime air strikes and artillery bombardment on rebel-held areas in eastern Aleppo, a monitor said citing a new toll.

The Syrian Observatory for Human Rights said Thursday's toll, up from an earlier figure of 16 killed, was one of the heaviest since the regime launched an offensive on eastern Aleppo on November 15.

"There was an escalation in the evening, with successive bombardments," Observatory head Rami Abdel Rahman told AFP, adding that many people were trapped under rubble.

"There are many wounded people, and bodies, still trapped under the rubble," he said, adding that several neighbourhoods had been hit across insurgent-controlled neighbourhoods.

An AFP correspondent said that the sound of the bombardment was deafening.

The reporter was able to travel to Al-Mashad neighbourhood where he saw rescuers sifting through the rubble of a building that had been hit, working in total darkness.

Three floors were flattened, the reporter said, adding that the building was located on a street that has been targeted in the past.

"We didn't realise what happened. Everything just came down all around us," said resident Ahmad.

At least 188 civilians, including 27 children, have been killed since the assault was launched on November 15, according to the Observatory. Rebel fire has killed 16 civilians in the government-held west, including 10 children.

Abdel Rahman said that Thursday's toll was "one of the heaviest of the past few days".

Earlier in the evening, Bab al-Nayrab, another neighbourhood in eastern Aleppo, was badly hit by what rescuers said was a barrel bomb dropped by a government helicopter.

An AFP cameraman captured video footage of rescuers battling from more than one hour to pull a seriously wounded boy from the rubble of a building.

- Pleading for help -

The lower part of the boy's body was stuck in the rubble and a bloodied gash could be seen at the back of his head as if part of the scalp had been ripped apart.

Rescuers used a pickaxe to hack away at the concrete in order to lift him out, the footage showed.

The boy, who could be heard crying out "Father, Father" as he pleaded for them to hurry up and get him out, was lifted alive from the rubble.

"The operation took us an hour, an hour and a half... and there is still a woman trapped under the rubble," one of the rescuers told AFP.

Since November 15, government forces have pounded eastern Aleppo with air strikes, barrel bombs and artillery fire in a bid to retake that part of the city from insurgents.

More than 250,000 civilians have been trapped under siege for months in rebel-held eastern Aleppo, with dwindling food and fuel supplies.

The battleground northern city, once Syria's economic capital, has been divided between the government-controlled western areas and the rebel-held eastern districts since 2012.

Overnight Tuesday, dozens of civilians tried to flee but were forced back by gunfire, the Observatory said.

The government offensive has hit hospitals and rescue centres and forced schools to close.

In neighbouring Idlib province, at least seven people were killed, including two women and two children, in air strikes on the town of Binnish, the Observatory said.

An AFP correspondent at the scene said one house had been levelled by the air strikes.

More than 300,000 people have been killed since Syria's war erupted in March 2011 with anti-government protests.