BEIRUT - A maternity hospital supported by Save the Children was bombed Friday in an air raid in Syria's northwestern Idlib province, causing casualties and damage, the Britain-based charity said.

The Syrian Observatory for Human Rights said a militant from Al-Nusra Front, which has changed its name to Jabhat Fateh al-Sham, was killed in the raid. "Save the Children supported maternity hospital in #Idlib bombed, casualties reported - numbers unconfirmed," the charity tweeted.

It said the bomb hit the entrance to the hospital. "This is the only hospital specialising in maternity and children in the northern western side of rural Idlib," a statement said. Alun McDonald, Save the Children regional media manager, said in a separate statement: "There are initial reports of some casualties among patients and staff, although numbers are so far unconfirmed."

The Britain-based Observatory, quoting local sources, said a militant was killed in the raid that struck the hospital in the rebel-held town of Kafar Takharim, leaving it heavily damaged and barely operational. It also reported that IS had executed 24 civilians after seizing a village in northern Syria.

Meanwhile, the UN on Friday urged Russia to give it control of humanitarian passages out of Aleppo in northern Syria where besieged residents cowered indoors afraid to use what some called "death corridors". Syrian regime ally Russia has announced the opening of three humanitarian passages for civilians wanting to flee rebel-held eastern districts of the city and for fighters to surrender.

But only a handful of Aleppo residents trickled out through one passage Friday, while others wanting to flee were turned back by rebels, the Syrian Observatory for Human Rights said.

"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 the monitoring group's head, Rami Abdel Rahman.

An AFP correspondent in east Aleppo said streets were empty in the morning with residents holed up indoors and shops shuttered.

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.

The Observatory said regime aircraft bombed eastern areas of Aleppo overnight.

UN Syria envoy Staffan de Mistura said Russia should 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?"

The UN says around 250,000 people have been trapped in Aleppo's eastern districts since July 17 when pro-regime forces surrounded the area.

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 which has not received any medical aid since July 7.

"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," said the International Committee of the Red Cross.

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 from the French think-tank IRIS said: "Aleppo residents are facing a terrible existential dilemma, they often have to chose between risking starvation or risking to die while fleeing."

"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."

Russian Defence Minister Sergei Shoigu announced Thursday that three humanitarian corridors were being opened "to aid civilians held hostage by terrorists and for fighters wishing to lay down their arms".

Losing Aleppo would be a major blow for the armed opposition and could signal a turning point in Syria's five-year-old 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 war, which erupted in 2011 with a brutal crackdown on anti-government protests before becoming a complex conflict and the spread of militants.

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.

Also on Friday, a maternity hospital supported by Save the Children was bombed in an air raid in Idlib province of northwest Syria, causing casualties and heavy damage, the Britain-based charity said.

It said there was an unspecified number of casualties.

The Observatory said the hospital in the rebel-held town of Kafar Takharim was heavily damaged and left barely operational.