Four World Food Programme workers who went missing in South Sudan over the last year are now presumed dead, the organization's executive director announced.

"Over the last year, four of our colleagues in South Sudan have disapeared without a trace. We have searched relentlessly for news of their whereabouts, hoping for the best but fearing the worst," said WFP Executive Director Etharin Cousin.

"After many months, we must sadly conclude that they are no longer alive," she said.

The four humanitarians were all South Sudanese nationals and disappeared in government-held parts of Upper Nile state.

One was abducted by gunmen in October of last year at the airstrip in Malakal town. The other three more went missing in April while traveling north from Malakal through Akoka county when clashes broke out.

As of late July this year 27 aid workers had been killed in South Sudan since the start of the conflict, according to the UN's top humanitarian official Stephen Obrien.

Related:

Three aid workers missing in South Sudan's Upper Nile (8 Apr.)

Four abducted UN staff still held in South Sudan (23 Oct.)