Two Sudanese employees of the Irish aid group GOAL are missing in the troubled Darfur region, a humanitarian source said on Friday.
They disappeared "following an incident that took place on 18th of June," said the source who asked not to be identified.
He could not give further details except to say the incident occurred near Kutum in North Darfur state.
"We are working very closely with the local authorities, with the Sudanese authorities, to gather more information and to find a solution," the source said.
Violence throughout Darfur has been at its worst in a decade this year, with about 395,000 people displaced since January, according to the United Nations.
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 has said, adding 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.