The recent Syrian government gains in Aleppo have prompted an exodus of thousands of desperate civilians, some fleeing to districts held by the government or Kurdish forces, others heading south into areas still under opposition control.

The opposition has steadily lost territory since Russia intervened to bolster President Bashar al-Assad in September 2015.

Syria’s White Helmets warned that they had no more fuel reserves for rescue vehicles.

In a video statement, the group urged “all humanitarian, aid, and medical organisations to immediately intervene to put an end to the humanitarian disaster” facing civilians in besieged Aleppo.

Nearly 10,000 civilians have fled the east, the Observatory said late Sunday, with about 6,000 moving to the Kurdish-held Sheikh Maqsoud neighbourhood and 4,000 to government-held west Aleppo.

Kurdish officials published a video they said showed civilians crossing a field to Sheikh Maqsoud, where local forces helped people cross a makeshift barrier.

The United Nations said it was “deeply concerned” about civilians in the east, where international aid is exhausted and food stocks are desperately low.

The UN has appealed for access to the east many times, but has failed to secure the necessary guarantees to enable aid deliveries.

“In terms of east Aleppo, we just need the green light from the people who control the roads going in because, as you know, the east of Aleppo is besieged,” said Ramesh Rajasingham, the UN’s deputy regional humanitarian coordinator for the Syria crisis.

The government advances mean the regime now controls at least a third of eastern Aleppo, just under two weeks into its renewed bid to recapture the city.

Regime bombardment of eastern districts killed 18 civilians, Abdel Rahman said.

The government assault of heavy air strikes, barrel bomb attacks and artillery fire has killed at least 247 civilians in east Aleppo, according to the Observatory.

Rebel fire into the government-held west has also killed at least 27 civilians, among them 11 children, since November 15, it says.

British Foreign Secretary Boris Johnson called for an immediate ceasefire in Aleppo, saying the assault “is threatening a humanitarian catastrophe”.

More than 400,000 people have been killed and millions displaced since the conflict began with anti-government protests in March 2011.