Initial field investigations and rapid tests conducted by the Sudanese health ministry had indicated a dengue fever outbreak. The tests followed earlier reports of a haemorrhagic fever outbreak in the area.

Sacha Bootsma, the WHO spokeswoman in Sudan, said the samples were sent to a WHO reference laboratory for further testing because of the possibility of cross-reaction between the different viruses of the same family.

In a statement, WHO announced: "Of 38 blood samples sent to the reference laboratory, Naval Medical Research Unit, NAMRU 3 in Cairo, 13 were confirmed positive for yellow fever."

The results found that the yellow fever virus was the causative agent of the ongoing epidemic and that the 75 deaths reported on 10 November were caused by yellow fever rather than dengue fever.

"Vaccination is the single most important measure in preventing this disease. This means a massive vaccination campaign has to be undertaken," Bootsma said on Monday.

"The total population of 1.8 million people above nine months old living in the region needs to be vaccinated immediately," she added. Affected localities included Dilling, Abu Gebeha, Kadugli, Talodi and Rashad.

The illness is spread through the bite of the Aedes mosquito and manifests various symptoms. While the symptoms may disappear after three to four days, some 15 percent of the cases may rapidly develop jaundice and suffer abdominal pains.

"We have immediately mobilised several teams of experts to assess case reporting and to provide technical assistance ... in order to interpret the epidemic pattern and trend," Bootsma said.

"We are providing the necessary insecticide and equipment to gain control of the situation," she added.

The UN Children's Fund (UNICEF) was providing drugs and medical supplies to support vector control activities, while NGOs in the field were supplying medical teams and volunteers.

"Case management capacity in all affected areas in South Kordofan needs to be strengthened to decrease the mortality rate," the WHO spokeswoman observed.

Bootsma noted that the current mortality rate was 29 percent. Patients needed to be treated urgently with proper medication in order to identify underlying infections, such as malaria, she said.

WHO said in a statement that the Sudanese health ministry had requested vaccines from the Global Alliance for Vaccines and Immunisation emergency vaccine stockpile.

"The request has been reviewed and accepted by the International Coordinating Group, and 1.7 million doses of yellow fever vaccine will be arriving in Sudan next week," the agency noted.

WHO and the UN Children's Fund (UNICEF) were supporting national authorities to ensure availability of vaccine and rapid implementation of the mass immunisation campaign, while Médecins sans Frontières had opened two treatment centres in Abu Gebeha and Kadugli.

Kadayapreth Ramachandran, UNICEF senior programme coordinator in Sudan, told reporters in Khartoum that the disease caused by yellow fever virus was difficult to recognise, especially in the early clinical stages.

Initially, yellow fever could easily be confused with malaria, leptospirosis, viral hepatitis and other viral hemorrhagic fevers, including dengue fever, he added.

The most recent incidence of yellow fever in the country occurred in May 2003 in the Imatong Mountains of southern Sudan. Seventeen cases were diagnosed, but no lives were lost.

The last yellow fever outbreak in South Kordofan was in 1940, during which 15,000 people were reportedly affected and 1,500 people died.