"Three ICRC colleagues and the Syrian Arab Red Crescent volunteer have been released and are safe and sound. We are waiting for further information about the other three colleagues," ICRC spokesman Ewan Watson told AFP.

Watson did not say who was thought to be responsible for the aid workers' abduction Sunday, which took place as their convoy drove from the northwestern Idlib province back to the Syrian capital Damascus after delivering medical aid.

The ICRC has not revealed details of the aid workers' nationalities, though it earlier said that most of the group were Syrian.