A spokeswoman for the U.N. World Food Program in Sudan says a six-person Bulgarian helicopter crew has gone missing after reports their aircraft made an unexpected landing in a conflict-torn area.

Amor Almagro said Wednesday the crew was on a contract with the agency's offices in South Sudan. She said it was not immediately clear why the helicopter made an unexpected landing in a remote area in South Kordofan, on the Sudanese side of the border with South Sudan, on Monday.

She said there are "strong indications" that the crew is safe, without elaborating.

Rebels in South Kordofan are fighting against Sudan's government, which has carried out aerial bombardments there.