The Swiss foreign ministry said it was aware of the situation and was in touch with the Red Cross and Congolese authorities.
The Red Cross has several offices in South Kivu, which like much of eastern DR Congo has been wracked by violence since the 1994 genocide in neighboring Rwanda spilled across the border.
"It is in order to protect and assist armed-conflict victims that we have been carrying out our activities in the area,'' Franz Rauchenstein, the head of the ICRC's mission in Dr Congo, said.
"We continue to insist that the strictly neutral, impartial and humanitarian nature of our work be recognised, and that our colleagues be able to return to their loved ones soon."
The Mai Mai group, which was formed to resist Rwandan Hutu forces who fled into DR Congo after the genocide, controls it own territory in the region.
The region has been the scene of violent clashes which have resulted in the displacement of thousands of civilians in recent months.
Staff of the aid group have previously been targeted for abduction in other conflict regions across the world.
Three foreign Red Cross workers were abducted in the Philippines last year and
French staff members were seized in Chad and Sudan. All have since been released.