Banerjee warned that yellow fever cases are "definitely spreading" to new areas of the remote region of Darfur, where Sudan's government has been battling rebel groups since 2003. More than 300,000 people have been killed in the conflict, and health care services are not available to many residents as a result of the turmoil.
He said that while no yellow fever cases have been found outside Darfur, the WHO is planning a risk assessment in the next two weeks on the assumption that all areas in Sudan may be at risk of infection.
Banerjee said that Darfur's heavy rainy season this year created additional breeding sites for the disease-carrying mosquitos.
Sudan's last outbreak of yellow fever killed 160 people in the South Kordofan region in 2005.