DAMASCUS, Nov 22 (Al Jazeera): UN humanitarian chief Stephen O'Brien, briefing the UN Security Council on Monday about the situation in Aleppo, said conditions in the rebel-held sector of the city had gone from "terrible to terrifying and are now barely survivable by human beings."
"Let me be clear, we are not just seeing a resumption of violence in Aleppo. This is not business as usual. What has been unleashed on civilians this past week is yet another low in an unrelenting, inhuman onslaught, and it is as heartbreaking as it is not inevitable," said O'Brien.
"These parties to the conflict are - all of them - choosing to do this. It is civilians who pay the price."
On Sunday, US President Barack Obama said that he was "not optimistic about the short-term prospects in Syria".
"Once Russia and Iran made a decision to back Assad in a brutal air campaign ... it was very hard to see a way in which even a trained and committed moderate opposition could hold its ground for long periods of time," he said.
Syrian government forces and allied fighters advanced further into rebel-held Aleppo on Monday, pressing an offensive in defiance of international concern for the fate of the city and its beleaguered civilians.
"At least 36 people were killed in Monday's bombing," rescue worker Ibrahim Abu Leith told Al Jazeera. "These are the most violent attacks we've seen in five years."
The recapture of the rebel-held east, which fell from government control in 2012, would be the government's most significant victory since the conflict began more than five years ago.
The international community appeared unlikely to halt the government's advance, despite expressing outrage over rising civilian deaths and the targeting of hospitals and rescue-worker facilities in the east.
Geert Cappelaere, regional director for the UN's children's agency, said more than 100,000 children were trapped. "Children should not be dying in hospitals because of bombs, and they should not be dying in schools."
Rebel forces have steadily lost ground since Moscow, a key backer of President Bashar al-Assad, intervened to bolster his government last year.
Activists and the UK-based Syrian Observatory for Human Rights said on Monday that government forces, backed by Iranian and Russian troops and fighters from Lebanon's Hezbollah, captured the eastern part of the Masakan Hanano neighbourhood.
"It is the most important advance inside the eastern neighbourhoods that the regime has made so far," said Syrian Observatory director Rami Abdel Rahman.
"If they take control of Masakan Hanano, the regime will have line-of-fire control over several rebel-held neighbourhoods and will be able to cut off the northern parts of rebel-held Aleppo from the rest of the opposition-held districts."
Abdel Rahman said that the advance had both strategic and symbolic significance, because Masakan Hanano was the first neighbourhood to fall to rebels in 2012.
Syria's Al-Watan daily, which is close to the government, described the neighbourhood as the "biggest and most important stronghold of the gunmen" in Aleppo.
On Sunday, Damascus rebuffed a United Nations truce plan for Aleppo that would see the east of the city temporarily administered by the opposition, saying it would "reward terrorists".