More than 25 of almost 230 suspected cases had been confirmed by laboratory analysis, WHO spokeswoman Fadela Chaib told journalists. She did not have any figures for the number of confirmed deaths.
The cases occurred in the White Nile, Sinnar and Gazeera states, south of Khartoum.
There are no vaccines for humans who contract Rift Valley Fever, usually via mosquitos, Chaib said.
Health experts generally advise the public to only eat inspected meat, avoid human-animal contact, bury or burn dead animals and observe basic hygiene.
Victims usually experience fever, generalized weakness, back pain, dizziness, vomiting of blood and extreme weight loss at the onset of the illness.
Many patients recover within a week but others can die, including those with weakened immune systems, experts say.
The fever was first isolated in Kenya's Rift Valley region in 1930s but has since been recorded elsewhere on the continent and abroad.