Elizabeth Hoff, WHO's representative in Syria, said Moaz and Nawaz had been safely transferred to the Damascus Children’s Hospital in the capital, which is predominantly under the control of Prsident Bashar al-Assad’s forces.
The organisation has a list of 16 critical medical cases in the town of Madaya, which is besieged by regime forces, and two adults in Foua and Kefraya – an area of Idlib province surrounded by rebel fighters.
Among those in Madaya is Ghina Ahmad Wadi, a 10-year-old girl who was shot by a sniper earlier this month while on her way to buy medicine for her mother.
Amnesty International said the wound to her thigh had severed a nerve and caused a complex bone fracture, leaving her in excruciating pain.
But Syrian authorities have so far denied requests to evacuate her to a hospital in Damascus or Lebanon where she can receive the treatment she needs.
Conjoined twins born in one of the most violent areas of Syria have been evacuated for life-saving surgery.
Baby brothers Moaz and Nawaz were born in 23 July in eastern Ghouta, a rebel-controlled area on the outskirts of Damascus that has seen vicious fighting, air strikes and chemical attacks.
Doctors at Zahra hospital feared the boys, who are joined at the chest with protruding intestines, would not survive without urgent surgery they were unable to provide.
In a letter on behalf of the Syrian American Medical Society, Dr Mohamad Katoub wrote that the hospital was undersupplied and “running out of options” after four years under a brutal siege.
“These parents have entrusted their children's case to us.
We must not let them down.” Dr Katoub called for the twins and their mother to be evacuated immediately so the possibility of surgically separating them can be explored.
With help from the World Health Organisation (WHO), the UN health agency, and Syrian Arab Red Crescent (SARC), the appeal was a success.