Six Bulgarian helicopter crew members assisting international aid efforts in Sudan and South Sudan have been released after being held by a rebel group for a week, the Bulgarian foreign ministry said.

The crew was forced to make an emergency landing on 3 January after coming under fire while flying over a remote area of South Kordofan state, the World Food Programme (WFP) said in a statement.

The helicopter was shot at and hit by the Sudan People's Liberation Army-North rebel group, the aircraft's captain Lubomir Gelev said, according to Nuba Reports.

The SPLA-N agreed to release the crew after talks involving the governments of South Sudan and Sudan, the WFP said.

The Bulgarians were delivered safely to a WFP office in Yida, a South Sudanese refugee camp near the border with Sudan, and were airlifted later on Sunday to South Sudan's capital Juba, the Rome-based UN agency said.

"It is regrettable that this incident took place. However we are greatly relieved that the WFP crew members are unhurt and back with us," WFP Executive Director Ertharin Cousin said.

The chairperson of New Sudan Women's Union in Yida camp Husna El Nur said the leadership of the SPLA-North in the liberated areas responded to an appeal by the women to release the Bulgarian crew.

“We delivered the crew and the employees of the World Food Programme on Sunday morning to the UN office at Yida camp,” she said.