Six Bulgarian contractors for the UN World Food Programme (WFP) who were held for a week by Sudan rebels have been freed, the Bulgarian foreign ministry said.

"The operation for liberating them ended. The six are in a safe place," ministry spokeswoman Betina Zhoteva said.

"The operation was performed in coordination between the Bulgarian government and the WFP. No ransom was paid and the case was not considered a kidnapping," she added.

The three crew and three Heli Air Services company officials were flying their UN-marked helicopter from South Sudan to the Sudanese capital Khartoum for maintenance last Monday when they made what the WFP called an "unexpected landing" in the southern Sudanese region of South Kordofan.

The reasons for the landing were unclear.

The six were then held by rebels of the Sudan People's Liberation Movement - North (SPLM-N), which has been fighting the Khartoum government since South Sudan split from Sudan in 2011.

Their Bulgarian company works on contract for the UN Humanitarian Air Service (UNHAS), a WFP-run operation providing air support to aid groups.