Pressing a new offensive in defiance of international concern over the fate of the city and its residents, more than 100 civilians have been killed in the east Aleppo since the Assad forces' latest offensive began on Tuesday, according to the Syrian Observatory for Human Rights monitor.

The U.S. President Barack Obama said he is "not optimistic" about Syria's future, as the U.N. warned time is running out to avoid a humanitarian catastrophe in Aleppo, which has been pounded by airstrikes for nearly a week.

Obama warned that Syria's second city was likely to fall, and that Russian and Iranian backing for Bashar Assad had made the situation untenable for the opposition. "I am not optimistic about the short-term prospects in Syria," he said Sunday at a summit of Pacific leaders in Lima. "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."

Obama earlier Sunday urged greater efforts to end the violence when he met Russian President Vladimir Putin at the Asia-Pacific Economic Cooperation forum. But in Damascus, U.N. envoy Staffan de Mistura was rebuffed on a truce proposal that would allow the opposition to administer the city's opposition-held east.

Aid agencies fear that instead of a humanitarian or a political initiative there will be "an acceleration of military activities" in eastern Aleppo and elsewhere, de Mistura 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."

The World Health Organization said there are no more functioning hospitals in east Aleppo, although it was impossible to confirm that independently. The agency said some health services were available through small clinics, but there was no longer access to trauma care or major surgery. The U.N. Security Council also meet yesterday in New York to discuss humanitarian efforts in Syria.

Aleppo is the focal point of the six-year war in Syria. Assad has said he is determined to retake the country's largest city and former commercial capital as his forces have maintained a siege on the opposition-held eastern quarters since September. The U.N. estimates 275,000 people are trapped inside with dwindling supplies of food and medicine.

Syria has been locked in a vicious civil war since early 2011, when the regime cracked down with unexpected ferocity on pro-democracy protests that erupted as part of the "Arab Spring" uprisings. Since then, more than a quarter of a million people have been killed and more than 10 million displaced across the war-torn country, according to U.N. figures.