Medical evacuations have begun from the Syrian opposition-held enclave of Eastern Ghouta to Damascus, the International Committee of the Red Cross (ICRC) in Syria said Wednesday morning.

Almost 400,000 people in Eastern Ghouta are besieged by Syrian regime forces and the United Nations has pleaded for the regime to allow the evacuation of around 500 patients, including children with cancer.

The Syrian American Medical Society (SAMS) said four patients were taken to hospitals in Damascus, the first of 29 critical cases approved for medical evacuation, and the remainder would be evacuated over the coming days.

SAMS advocacy manager Mohamad Katoub said in a tweet that five had been approved for the first group of evacuations, part of a deal on an exchange of detainees between the regime and opposition group Jaish al-Islam. It was not clear why only four of the five had left.

The Syrian Red Crescent said the evacuations were the result of long negotiations. An ICRC spokeswoman declined to give more details, citing the sensitivity of the operation.

On Sunday, President Recep Tayyip Erdoğan said Turkey was working with Russia, Bashar Assad's ally, on the evacuations.

Last week, Jan Egeland, the United Nations humanitarian adviser for Syria, said 494 people were on the priority list for medical evacuations.

"That number is going down, not because we are evacuating people but because they are dying," he said. "We have tried now every single week for many months to get medical evacuations out, and food and other supplies in."

The United Nations has been waiting for months for the Syrian authorities to provide facilitation letters to allow the aid operation to get under way.

Forces loyal to Assad have carried out air raids on Eastern Ghouta while besieged opposition fighters have fired mortars into neighboring Damascus.

"That has nothing to do with the right of evacuating, and obligation to evacuate civilians [and] wounded," Egeland said.

At least 16 people have died while waiting for medical evacuation from Eastern Ghouta, the United Nations said last Thursday. "We have confirmation of 16 having died on these lists since they were resubmitted in November, and it is probably higher," he said, highlighting the case of an infant who died on Dec. 14, as the latest round of Syria peace talks in Geneva ended in failure.