Russian fighter jet
Russian fighter jetReuters

Syria's army took control of a rebel-held district in central Aleppo on Tuesday, after days of heavy air strikes that have killed dozens and sparked allegations of war crimes.

In the first advance since announcing plans last week to retake all of the divided city, pro-government troops seized the Farafina district northwest of Aleppo's historic citadel, a military source told AFP.

"After neutralising many terrorists... units are now demining the area," the source said.

The push follows several days of Syrian and Russian air strikes on rebel-held Aleppo neighbourhoods - some of the fiercest bombardment of the five-year conflict so far - after a ceasefire deal brokered by Moscow and Washington collapsed last week.

The Aleppo maelstrom prompted Western powers to accuse Russia of committing possible war crimes, charges the Kremlin condemned as "unacceptable".

In the latest broadside, NATO Secretary General Jens Stoltenberg also condemned the air campaign.

"The appalling attacks on Aleppo have shaken all of us, and the violence and the attacks we have seen... is morally totally unacceptable and is a blatant violation of international law," Stoltenberg told a news conference in Bratislava.

On the ground in eastern Aleppo, an AFP correspondent said air strikes struck several neighbourhoods simultaneously, including in Al-Shaar, where a five-storey building was levelled with a family stuck inside.

One young girl, her body encased in rubble, was among the dead. Her father, in shock as rescue workers picked up her lifeless body, collapsed beside, saying: "She's just sleeping. She's just used to sleeping."

The Syrian Observatory for Human Rights, a monitor, said that more than 155 people have been killed by Russian and regime bombardment on Aleppo city since the government announced its offensive last week.

They include 11 civilians killed Tuesday in raids on the neighbourhoods of Al-Shaar and Al-Mashhad, according to the Britain-based group.

As well as the intensified violence, residents have been left reeling from food shortages and skyrocketing prices.

The World Health Organization warned Tuesday that medical facilities in east Aleppo were on the verge of "complete destruction".

"Over the last weekend alone, more than 200 people were injured and taken to understaffed health facilities in east Aleppo," a spokeswoman said in Geneva.

The UN body called for "an immediate establishment of humanitarian routes to evacuate sick and wounded from the eastern part of the city."

'Absolutely unacceptable'

The Observatory said that there were "significantly fewer" strikes on Aleppo on Tuesday than in recent days, but confirmed the advance by pro-government forces into Farafina.

Aleppo has been roughly divided between government control in the west and rebel control in the east since mid-2012, and the frontline has remained largely static despite continuous violence.

Earlier this month, a ceasefire went into effect across Syria, brokered after exhaustive talks between US Secretary of State John Kerry and his Russian counterpart Sergei Lavrov.

It was dubbed by Kerry as the "last chance" to end more than five years of devastating conflict.

But it fell apart within a week, with each side blaming the other for the latest failure in a war that has cost more than 300,000 lives since March 2011.

Analysts said Tuesday that the unprecedented ferocity with which Aleppo has been hit in recent days suggested that Moscow was backing the Syrian government's aim to totally recapture the city.

"Russia has decided to go all out because it no longer believes in the possibility of collaborating with the United States in Syria," said Fabrice Balanche, a Syria expert at the Washington Institute for Near East Policy.

At an emergency session of the UN Security Council on Sunday, US ambassador Samantha Power accused Russia of "barbarism", while both the British and French envoys went further, alleging the bombing of Aleppo constituted possible war crimes.

German Chancellor Angela Merkel said Tuesday that violence in Aleppo was on an "absolutely unacceptable scale" and that it was up to Russia and Syria to ensure humanitarian aid could reach the battered city.