Heavy artillery exchanges rocked Syria's battleground second city Aleppo throughout the night as government forces pressed a two-pronged assault on rebel-held districts, an AFP correspondent reported on Saturday.

The government's offensive to recapture the whole of Aleppo has been bolstered by its Russian ally's rejection of Western demands for a halt to its deadly bombing campaign in support of the advancing troops.

There have been mounting civilian casualties on both sides of the divided city. About 250,000 residents are living under siege by the army in the rebel-held east, and around 1.2 million face daily rocket fire by the rebels on the government-held west.

Medical charity Doctors Without Borders has described the impact of Syrian and Russian bombardment of the east as a ‘bloodbath.’

Six children were among 20 civilians killed in the rebel-held sector on Friday, the Syrian Observatory for Human Rights said.

More than 220 people have been killed in the east of the city since the government launched its offensive on September 22, the Britain-based monitoring group said.

In west Aleppo, rebel rocket fire killed 15 civilians and wounded 40 on Friday, state television reported.

The fighting saw the army advance in both the Suleiman al-Halabi neighbourhood in the city centre and the Bustan al-Basha district in the north, the Observatory said.

Residents of government-held neighbourhoods expressed relief that the rebels were being pushed back but said they feared retaliation.

‘We were happy when we heard about the army's advance,’ said Majed Abboud, a 32-year-old car dealer.

‘But I'm afraid that with these ferocious clashes, there will be some kind of reaction from the armed groups,’ he said.

‘They hit us with rockets and there were many casualties today (Friday) in Suleiman al-Halabi and Al-Midan.’

The offensive has seen the army win back territory in the north of Aleppo it had not held since 2013.

Troops have also pushed back the front line in the city centre which had remained largely static since the rebels seized eastern districts in 2012.

But the civilian death toll has triggered mounting outrage in the West against both the regime and its Russian ally.

British Foreign Secretary Boris Johnson told Saturday's Sun newspaper that Moscow risked becoming an international pariah.