Aleppo - Aid agencies urged Syria's regime to end the siege of rebel-held east Aleppo as residents cowered indoors on Friday afraid to use what some described as "death corridors" to flee the city.Russia, a key ally of President Bashar Assad, announced on Thursday the opening of humanitarian passages for civilians and surrendering fighters seeking to exit the city's rebel-held eastern neighbourhoods.By Friday only a few Aleppo residents had left eastern areas through one passage but others wishing to flee were prevented by rebels, said the head of the Britain-based Syrian Observatory for Human Rights."Around 12 people managed to use the Bustan al-Qasr corridor before rebel groups reinforced security measures and prevented families from approaching the corridors," said Rami Abdel Rahman.An AFP correspondent in east Aleppo said streets were empty on Friday morning, with residents holed up indoors.Shops were shuttered and generators in several neighbourhoods had stopped after their fuel ran out.Ahmad Ramadan from the opposition Syrian National Coalition accused Russia and the regime of forcing civilians to flee through continued bombing raids."Aleppo residents are calling the corridors that Russia is talking about 'death corridors'," he said."What is happening now is not battles, but the complete and systematic destruction of the city and its residents, whether they are civilians or fighters."The Observatory said regime aircraft bombed eastern areas of Aleppo overnight.On Friday, UN Syria envoy Staffan de Mistura urged Russia to let the United Nations take charge of the corridors as a reassurance to the beleaguered population."Our suggestion is to Russia to actually leave the corridors being established at their initiative to us," he said."How can you expect people to want to walk through a corridor, thousands of them, while there is shelling, bombing, fighting."Pro-regime forces have surrounded Aleppo's eastern districts since July 17, leaving an estimated 250 000 trapped without reliable access to food or medical aid.Residents have reported food shortages and spiralling prices in rebel districts since regime forces cut off the opposition's main supply route into the northern city. Aid agencies and analysts said the humanitarian corridors must be exploited to send desperately needed supplies to the besieged areas.'Existential dilemma' "Those who decide, for whatever reason, to stay in eastern Aleppo must be protected, and all sides must allow humanitarian agencies to reach and assess their well-being and needs," the International Committee of the Red Cross said. The US-based International Rescue Committee warned that those left behind in east Aleppo risked starvation and called for a pause in fighting."The people of Aleppo should not be forced to choose between fleeing their homes and remaining under attack in a besieged area," said IRC's acting Middle East director Zoe Daniels.Analyst Karim Bitar added: "Aleppo residents are facing a terrible existential dilemma, they often have to chose between risking starvation or risking to die while fleeing.""If the objective is really to ensure the safety of Aleppo residents, why aren't aid workers and ICRC getting sufficient access to the civilians in dire need of protection?" he asked"Aleppo residents are in distress and mistrustful, which is understandable, as the Syrian tragedy has shown that even humanitarianism is often used as a cynical ploy to advance geopolitical interests," said Bitar, from the French think-tank IRIS.Russian Defence Minister Sergei Shoigu has said three humanitarian corridors were being opened "to aid civilians held hostage by terrorists and for fighters wishing to lay down their arms".'Rebel-civilian dynamic' Losing Aleppo would be a major blow for the armed opposition and could signal a turning point in the five-year conflict, analysts say."In Aleppo, getting civilians to leave would both serve its propaganda and its military objectives," said Emile Hokayem, senior fellow at the International Institute for Strategic Studies."The regime uses massive, indiscriminate force to brutalise civilians to force them to kneel or reject the rebel groups," he added.More than 280 000 have been killed in Syria's devastating war which erupted with the brutal crackdown of anti-government protests before becoming a complex conflict that has seen the spread of jihadist.A US-led coalition is conducting an aerial campaign against the Islamic State group, which despite battlefield losses still controls areas of north and northeastern Syria.The Observatory coalition strikes on Thursday on the ISIS-controlled town of Ghandoura has killed 28 civilians.It also reported that ISIS has executed 24 civilians in a village close to Manbij.