Obama says Russian and Iranian involvement has made a viable opposition impossible as fighting continues on the ground.

US President Barack Obama is “not optimistic” about Syria’s future and said the chaos could persist for “quite some time” as the United Nations warned that time was running out to avoid a humanitarian catastrophe in the city of Aleppo, which has been pounded by air strikes for nearly a week.

Obama, who will be succeeded on January 20 by Donald Trump, said that he told Russian President Vladimir Putin on the sidelines of the Asia-Pacific Economic Cooperation summit in Peru that he was deeply concerned about the bloodshed in Syria and that a ceasefire was needed.

He warned that Syria’s second city was likely to fall, and that Russian and Iranian backing for Syrian President Bashar al-Assad had made the situation untenable for the opposition.

“I am not optimistic about the short-term prospects in Syria,” he said at a summit of Pacific leaders in Lima.

“Once Russia and Iran made a decision to back Assad and a brutal air campaign and essentially a pacification of Aleppo regardless of civilian casualties, children being killed or wounded, schools or hospitals being destroyed, 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.

On the ground in Syria, government forces launched a ferocious assault last Tuesday to recapture eastern Aleppo, killing 115 civilians so far.

In fresh fighting on Sunday, at least eight children were killed when rebel rocket fire hit a school in the government-controlled west Aleppo.

In Damascus, UN envoy Staffan de Mistura 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 after meeting Syrian Foreign Minister Walid Muallem.

OPINION: I live in Aleppo, under siege

The Syrian foreign minister said he had rejected the proposal under which rebel forces would leave and the government would recognise the opposition administration in the east which has been bombarded by air strikes, barrel bombs and artillery.

“How is it possible that the UN wants to reward terrorists?” Muallem asked.

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, according to de Mistura.

“By Christmas, due to military intensification, you will have the virtual collapse of what is left in eastern Aleppo,” he said.

“You may have 200,000 people moving towards Turkey and that would be a humanitarian catastrophe.”

The latest warnings from western leaders come as rebel forces add to the indiscriminate shelling of the Syrian government forces.

Sunday’s attack on a school that killed eight children came after at least 19 civilians, including five children, were killed in the east, according to the UK-based Syrian Observatory for Human Rights.

UN chief Ban Ki-moon condemned the indiscriminate shelling, saying that it had killed and maimed civilians, destroyed schools and left the city’s east without functioning hospitals.

“The Secretary-General reminds all parties to the conflict that targeting civilians and civilian infrastructure is a war crime,” his office said in a statement.

“Those responsible for these and other atrocities in Syria, whoever and wherever they are, must one day be brought to account.”

Friday’s shelling destroyed one of the last hospitals in east Aleppo, and staff were also forced to evacuate the area’s only children’s hospital because of repeated attacks.

Russia, which intervened militarily last year, says it is not involved in the current assault on Aleppo, and is instead concentrating its firepower on opposition forces in neighbouring Idlib province.