GENEVA, Nov 22 (Reuters) - An outbreak of yellow fever has killed at least 121 people in Sudan since Nov. 10, and the World Health Organisation is organising the emergency dispatch of 1.7 million vaccine doses to prevent the spread of the disease.

The WHO said on Tuesday it feared the outbreak of the mosquito-borne disease in South Kordofan state in central Sudan could spread rapidly in rural areas where people were poor, nomadic and unvaccinated.

Sudan, which has reported a total of 448 cases, asked for the vaccines from the emergency stockpile of the Global Alliance for Vaccines and Immunisation, the WHO said, adding that the does would arrive next week.

"There is no immunity against the disease in the region, where it is not endemic and there is no history of vaccination over the past 10 years," spokeswoman Fadela Chaib told Reuters. "The risk of rapid spread appears high."

Heavy rains have allowed "aedes aegypti" mosquitoes to multiply and spread the viral haemorrhagic fever.

A WHO expert leaves on Wednesday for Sudan and will be joined in the coming days by five more international experts, WHO spokeswoman Maria Cheng said.

"They will help with risk assessment and determine what should be done in the next weeks. There are a lot of nomadic people and they have to assess migration routes to see the best points to catch people (for vaccination)," Cheng said.

"The health infrastructure is not strong and in a lot of cases non-existent," she added.

South Kordofan is one of the areas which was gripped by a brutal civil war which raged for all but 11 years since Sudan's independence from Britain in 1956. Health care systems are basic, with only limited access for its rural population.

Earlier this month a senior Sudanese Health Ministry official said dengue fever had claimed 71 lives so far out of 299 suspected cases in the South Kordofan region.