Syrian staff members from the International Committee of the Red Cross take part in a medical evacuation operation in Eastern Ghouta, a besieged rebel bastion on the outskirts of Damascus on Dec. 26. The area has been under government siege since 2013, with shortages now reaching a crisis point. (Amer Almohibany/AFP/Getty Images)

The Red Cross said Wednesday that it has begun an evacuation of critically ill civilians from a besieged suburb of the Syrian capital as part of a rare deal struck between the government and a rebel group.

Ambulances conveyed three sick children and a woman out of Eastern Ghouta late Tuesday for medical treatment in Damascus hospitals. Some 400,000 other people remain trapped in the district, a pocket of opposition-held territory where a long-running government blockade has caused scores of civilians to die of starvation or a lack of medical care.

Under the terms of a deal between Syrian President Bashar al-Assad's government and the hard-line Jaish al-Islam rebel group, which controls Eastern Ghouta, 25 more people are expected to be evacuated for lifesaving medical treatment in the coming days.

The International Committee of the Red Cross described the evacuation as a "positive step" that it hoped would be followed by more. 

But the medical mission also underscored the extent to which hundreds of thousands of civilians in Syria's remaining rebel-held enclaves now rely on the dealmaking of belligerents still slugging it out in the dying stages of a war that has claimed almost a half-million lives.

The evacuation is part of a wider agreement that also involved the exchange of 29 government soldiers held by Jaish al-Islam.

Mohamed Kattoub, a spokesman for the Syrian American Medical Society, said the 29 patients slated for evacuation were drawn from a list of more than 600 sick civilians submitted to the United Nations in October. How they had been selected remained unclear, he said.

In the intervening weeks, scores of patients have been treated in underequipped hospitals for severe malnutrition, now reaching a crisis point after years of siege. Medical facilities in the area have repeatedly been bombed by government warplanes, according to local doctors. 

"In the two months since we submitted the list, we have added the names of many more patients with life-threatening conditions like meningitis, or with wounds that cannot be treated in the hospitals we have left," said Hamza Hasan, a doctor in Eastern Ghouta.

Rights groups say the government has used siege tactics across Syria to starve local populations and force rebels to surrender, which would amount to a war crime. The government denies the allegations, blaming the shortages on rebel groups controlling each area.

After six years of war, Syrian government forces appear to be wrapping up their campaign, mounting offensives against ­opposition-held pockets in the country's north and center. 

Civilians in Eastern Ghouta voiced concern Wednesday that the families cleared for evacuation had been given no guarantees that they would be allowed to return to their homes or that they would not be arrested upon departure. 

The United Nations has repeatedly called on the government to allow hundreds of critically ill people to leave the suburb for treatment and to expand aid groups' heavily restricted access to the area. With food supplies running low and temperatures plunging, activists have also circulated pictures of children so thin that their ribs strain against their flesh.

A local charity worker said Wednesday that at least one of the patients on the evacuation list, a 6-month-old girl called Nada al-Taweel, had died of heart failure in the hours before the ambulances arrived. 

The aid worker gave her name as Laila, asking that her family name be withheld for fear of government retribution.

Louisa Loveluck in Beirut contributed to this report.