Aid workers completed a series of medical evacuations from a besieged rebel enclave near Damascus Friday as part of a controversial deal that saw patients swapped for hostages and prisoners.

The patients were among a list of cases considered critical who were evacuated since Tuesday night from Eastern Ghouta, an area where the humanitarian crisis has escalated in recent months."Thirteen civilians, including six children and four women, were evacuated" during the night of Thursday to Friday, a health official in Eastern Ghouta told AFP on condition of anonymity.

The latest batch of patients the Syrian Red Crescent evacuated brought to 29 the number of civilians who were able to leave the area, which has been virtually cut off from the outside for four years.

The 29, 17 of them children, were deemed the most pressing cases on a list of around 500 people the United Nations said last month could die if they did not receive urgent assistance outside the besieged enclave.

Humanitarian access to the area, which lies just east of the capital Damascus, has been very difficult and only limited convoys of aid have reached it in recent months.

A crowd of residents gathered at night around the ambulances to see their relatives and neighbours one last time.

Red Crescent nurses attended to Marwa, a 26-year-old woman suffering from meningitis who was being stretchered onboard and given respiratory assistance.

Among the patients who made it out were Fahed al-Kurdi, a 30-year-old man with cancer, and Zuheir Ghazzawi, a 10-year-old boy who also has cancer and had a leg amputated.

- 'Without conditions' -

Another 16 patients had been evacuated on Tuesday night and Wednesday night by the Red Crescent and the International Committee of the Red Cross.

According to the Syrian Observatory for Human Rights, the patients were evacuated as part of a deal that saw the rebels who control Eastern Ghouta release hostages and prisoners.

There were also 29 of them, according to the head of the Britain-based Observatory, Rami Abdel Rahman.

Among them were some workers who were detained earlier this year as well as pro-government fighters whom rebels from the Jaish al-Islam group captured, in some cases years ago.

The deal raised concern that sick civilians were being used as bargaining chips.

"If they exchange sick children for detainees that means children become bargaining chips in some tug of war," Jan Egeland, head of the Norwegian Refugee Council and currently a UN special envoy for humanitarian access in Syria, told the BBC.

The ICRC issued a statement on Friday urging the parties involved to allow for more evacuations and better humanitarian access.

"The evacuation was a positive step to end the immense suffering of some people in Eastern Ghouta, especially children who have limited access to life-saving medical care," ICRC Syria head of delegation Marianne Gasser said.

"But more needs to be done. The needs of civilians should come first, be it in Ghouta or elsewhere in Syria, and access to aid should be allowed on a more regular basis and without conditions," she said.