South Sudan has been listed as the most violent country to deliver aid to for a third time in a row, the Norwegian Refugee Council (NRC) has said.

One in three of the 158 major violent incidents against aid operations that took place last year occurred in the East African nation.

About 100 aid workers have lost their lives since the civil war broke out in December 2013, with 24 aid workers dying of gunshot wounds last year.

Sudanese staff have borne the brunt of the attacks.

They often work in the hardest-to-reach locations, which can also be the most dangerous, NRC says.

Syria, Afghanistan and the Central African Republic were listed the next most dangerous, followed by Nigeria and Somalia.

Jan Egeland, the secretary-general of NRC, said the organisation was "cautiously optimistic" about the recent signing of a ceasefire deal between President Salva Kiir and his rival, Riek Machar.