Conjoined twin baby boys in Syria were being evacuated across lines from a rebel stronghold to Damascus Children's Hospital on Friday, the first of at least 20 patients who need urgent transfers to be saved, the World Health Organization (WHO) said.

Moaz and Nawras were born conjoined on July 23 in Zahra hospital in eastern Ghouta, a rebel bastion and rural suburb of the capital. Syrian doctors abroad sought help from the WHO, the United Nations health agency.

"The hospital is under-supplied and unable to provide the twins with the surgery they need to survive," said a letter by the Syrian American Medical Society.

"The twins, the mother and the aunt are now being evacuated to the Children's Hospital (in Damascus)," Elizabeth Hoff, WHO's representative in Syria, who has been based in Damascus throughout most of the country's five-year civil war, told Reuters on Friday:

An ambulance of the Syrian Arab Red Crescent (SARC) was transporting them, she said.

"We have been negotiating for medical evacuation for some days now," Hoff said by telephone.

The WHO also wants to get seriously ill and wounded patients out of the divided city of Aleppo, where up to two million people are trapped. The eastern, rebel-held part of the city has been besieged by Syrian government forces with Russian support for months, but earlier last week rebel forces broke the siege and opened a corridor into the city.

The United Nations said on Thursday it was talking to Russia about a "workable humanitarian pause" in fighting in Aleppo and that the three hours a day proposed by Moscow was "not enough".

"WHO is calling for a pause for medical evacuations which are critical to make sure seriously wounded have the right to obtain health care and also people with chronic diseases needing regular care," Hoff said.

In addition, WHO has received a list of 16 critical medical cases in the government-besieged town of Madaya who need evacuation and two adults in the opposition-besieged Foua and Kefraya area, in Idlib province, she said.

"We haven't obtained permission yet to evacuate," she said, noting that Hezbollah fighters were part of the mix in the Foua area.

Staffan de Mistura, U.N. special envoy for Syria, read out the names of the patients in Madaya and Foua, most of them young children, on Thursday to reporters in Geneva.

"The UN is ready to evacuate them. They are in a desperate urgent medical emergency. Why on earth should this not be possible?

"This should not be waiting for the Aleppo ceasefire or overall ceasefire, this should and can be done before it's too late," he said.

In Foua, the Islamist rebel group Ahrar al Sham "can just by a decision of a minute allow the U.N. to evacuate them and bring them to a medical facility where they could be saved", he said.

"These are not numbers, these are people who are waiting for a medical evacuation in what has become a medieval approach to a conflict," de Mistura said.