The UN's envoy warned Sunday that time was "running out" for efforts to avoid a humanitarian catastrophe in Syria's war-battered Aleppo, as fresh fighting there killed at least eight schoolchildren. Staffan de Mistura met Foreign Minister Walid Muallem in Damascus for talks on the escalating violence, but was rebuffed on a truce proposal that would allow the opposition to administer the city's rebel-held east.
"We are running out of time, we are running against time," De Mistura said afterwards.
Aid agencies fear that "instead of a humanitarian or a political initiative" there would be "an acceleration of military activities" in eastern Aleppo and elsewhere, he told journalists.
"By Christmas... due to military intensification, you will have the virtual collapse of what is left in eastern Aleppo; you may have 200,000 people moving towards Turkey - that would be a humanitarian catastrophe."
International concern has been mounting since Damascus began a ferocious assault last Tuesday, using air strikes, barrel bombs and artillery fire in a bid to recapture eastern Aleppo.
On Sunday, rebels retaliated with a barrage of rockets into the city's government-held west, killing at least eight primary school children in the Furqan neighbourhood, state media said.
Syrian television showed bloodied, weeping children being treated in hospital, and an AFP journalist saw pupils being rushed from the school and comforted after the attack.
In the east, another AFP journalist said streets were deserted, with only ambulances and rescue workers moving around.
The Syrian Observatory for Human Rights, a Britain-based monitoring group, said on Sunday that 54 people had been killed in the previous 24 hours, mostly civilians.
That brought to 103 the number of civilians killed, including 17 children, since the bombardment of east Aleppo resumed, it said.
The Observatory also reported heavy fighting between regime forces and rebels as the army sought to gain ground in the eastern Bustan al-Basha and Sheikh Saeed neighbourhoods.
More than 300,000 people have been killed in Syria since the conflict began with anti-government protests in March 2011. Successive international attempts to find a peaceful resolution have failed.
De Mistura said he opened his talks with Muallem by "expressing serious concern and indeed shared the general international outrage for the news coming from eastern Aleppo".
Muallem said he had rejected a proposal for a truce deal that would recognise an autonomous rebel administration in east Aleppo.
De Mistura has recently floated a proposal to halt fighting in the city, under which jihadist forces would leave and the government would recognise the opposition administration in the east.
But Muallem said that was a non-starter.
"We told him that we reject that completely," he said. "How is it possible that the UN wants to reward terrorists?"
The regime offensive has forced the closure of hospitals and schools, destroyed rescue worker facilities and left residents cowering in their homes.
US National Security Advisor Susan Rice said Washington condemned "in the strongest terms these horrific attacks against medical infrastructure and humanitarian aid workers".
"The Syrian regime and its allies, Russia in particular, bears responsibility for the immediate and long-term consequences these actions have caused in Syria and beyond."
Moscow intervened militarily in support of President Bashar al-Assad's government last year.
It says it is not involved in the current assault on Aleppo, concentrating its firepower on opposition and jihadist forces in neighbouring Idlib province instead.
But Damascus and its allies have made clear they want rebels expelled from eastern Aleppo, which fell from regime control in mid-2012.
More than 250,000 people remain in eastern Aleppo, which has been sealed off since government forces surrounded it in mid-July.
No aid has entered the east since then, and the siege has created food and fuel shortages.
In mid-October, Russia said it was halting its strikes on Aleppo, and organised a series of brief cease-fires intended to encourage civilians and surrendering rebels to evacuate the east.
But few did so, and the UN said the short windows were insufficient for it to secure security guarantees for aid deliveries or evacuations.
The renewed bombing has particularly affected medical and rescue facilities in the east, and shelling on Friday destroyed one of the last hospitals in rebel-held Aleppo.
Staff were also forced to evacuate the east's only children's hospital because of repeated attacks.
Among those killed in bombing on Saturday were a couple and their four children who died in a barrel bomb attack in the Sakhur neighbourhood, the Observatory said. Activists circulated footage that they said showed the four siblings lying lifeless on a stone floor.