Humanitarian organisations say 400,000 people face starvation as they are trapped in rebel-held eastern Ghouta in need of aid.
The first Red Cross ambulance carrying seriously ill patients, including children with cancer, was allowed to leave a rebel-held enclave in Damascus.
The Syrian government allowed the evacuation after a prisoner exchange deal with the rebels.
The first four patients have been discharged for treatment in local hospitals and in the next few days will follow 25 others.
These are the ones identified as requiring the most urgent treatment.
The eastern Ghouta has been besieged by the Syrian army since 2013 and is one of the few areas in the country still under rebel control.
International humanitarian organizations fear that 400,000 people are still trapped in famine facing famine.
Francesco Rocca, president of the International Federation of Red Cross (ICRC) told Sky News that the situation inside Ghouta is crucial.
He said: "The situation is very difficult. Our last humanitarian access was at the end of November and we bought goods for 7,000 but there are more than 400,000 people in Eastern Ghouta.
"This is the reason why what is happening now is a sign of hope. On the other hand, we continue to ask for unhindered access to this area because the level of suffering has really reached a critical point.
"They are going to be treated in the Damascus children hospital with all the assistance of the medical doctors of the hospital and the staff of the Red Cross and families are there to verify that and if there is a need to move the children even abroad then we will do it.
"At the moment we have decided that the Damascus hospital was the best place in which they could be treated."
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wakan min bayn aldhyn tama 'iijlawuhum thlatht 'atfalun, yubalighun min aleumr sanatan wahidat, watifl bialgha. yahtaj almaradaa, aladhin safaruu mae 'afrad aleayilati, 'iilaa eilaj fawri lilsurtan walfashl alkulawii walnuzf.
waqad la yatamakan bed almardaa min mughadarat alghawtat alsharqiat fi almanatiq alkhadieat lisaytarat alhukumati, li'anahum 'iimaa yakhshawn altajnid fi aljaysh 'aw alaihtijaz bsbb eayshihim 'aw aleamal fi manatiq muearadat.
rahab hamish di baritun jurdun, mustashar aitihad alrieayat altibiyat wamunazamat al'iighathati, bieamaliat al'iijla'i, likunh hadhar min 'anah yataeayan alqiam bialkathiri.
waqala: "hdha hu basis 'amal saghirin, wanahn fi munaqashat mae alrws walnizam alsuwrii limuhawalat tamdid waqf 'iitlaq alnnar hataa yumkin alhusul ealaa almusaeadati, wayumkin 'aydaan 'an tabda 'iieadat albna'.
Three children, one-year-olds and an adult child, were among the evacuees. Patients who have traveled with family members need immediate treatment for cancer, kidney failure and bleeding.
Some patients may not be able to leave the eastern Ghouta in government-controlled areas because they are either afraid of being drafted into the army or detained for living or working in opposition areas.
Hamish de Breton Gordon, adviser to the Union of Medical Care and Relief Organizations, welcomed the evacuations, but warned that much needed to be done.
He said: "This is a glimmer of small hope, we are in discussions with the Russians and the Syrian regime to try to extend the cease-fire so that aid can be obtained, reconstruction can also begin.
"The Russians and Syrians have won the war and together with UK and US they have military defeated IS.
"What is now needed is massive amounts of aid to get to the people - 11 million displaced - and start reconstruction.
"The British Government has already allocated more than £ 1bn to do this. [Boris] Johnson has previously suggested British troops could have been part of UN peacekeeping.
"We can enable reconstruction while the political process goes on. With reconstruction and aid means the political process will hopefully allow free and fair elections in next 18 months. So there is a ray of hope today."
For weeks, the UN has been calling on the government to allow some 500 critically-ill people to leave the suburb for treatment and to expand aid groups' heavily restricted access to the area.
Activists in eastern Ghouta have circulated photos online of severely malnourished children.
The UN says one in eight children in the district is going hungry, up from one in 50 in May.