Irish aid agency GOAL has said it is working with the relevant authorities in Sudan to locate two of its Sudanese aid workers.
They are among three people missing following an incident in the Darfur region earlier this week.
GOAL has said three of its local staff members and a Sudanese national working with another agency were abducted by an unknown group while travelling by vehicle on Wednesday.
The driver of the vehicle, a GOAL staff member, was later released, but the other three aid workers remain unaccounted for.
Violence throughout Darfur has been at its worst in a decade this year.
The United Nations has said that 395,000 people have been displaced since January.
Peacekeepers have blamed government paramilitaries, inter-communal fighting, rebel offensives and criminal activity for the surge in unrest.
Over the past two years as Sudan's economy weakened, inter-communal clashes over resources intensified in Darfur.
UN chief Ban Ki-moon said that tribal militias have become involved.
Analysts say the government can no longer control its Arab tribal allies, whom it armed against insurgents who began a rebellion in 2003.