Almost 4-million Sudanese have been forced from their homes in 14 years of tribal violence and battles between government and opposition fighters, the Norwegian Refugee Council says.
The aid group listed Sudan as number three in its latest report, The World's Most Neglected Displacement Crises, saying almost five-million Sudanese were now dependant on humanitarian aid, with three-million of those living in the war-scarred region of Darfur.
"Hundreds of thousands of people do not receive the lifesaving help they need because of challenges in accessing communities," the council said before blaming "a 40 percent shortage in funding and a lack of international media attention to the crisis".
Jan Egeland, secretary-general of the Norwegian Refugee Council, said: "The international community has not only forgotten these crises, but has never really shown sufficient willingness to contribute to a solution.
"Many of the displaced people have fled their homes multiple times, and each time they get increasingly vulnerable."
Darfur has been unstable since 2003 when different groups complaining about marginalisation by the Sudanese government started an armed campaign.
The conflict attracted a government-backed militia known as the Janjaweed, which were blamed for widespread killings that have been labelled as genocide.
Babar Baloch, a UNHCR spokesman, told Al Jazeera: "You have a situation where people are running for a conflict in South Kordofan [Sudan] and coming to South Sudan.
"But South Sudan is also in conflict, there is famine, more than half of the population is facing food insecurity and a quarter of them are displaced, including in Darfur and Kordofan."
The UN estimates as many as 300 000 people have been killed in the Darfur conflict, but the Sudanese government says the death toll has been grossly inflated.