GENEVA (AP) — A top U.N. health agency official says he expects some increase in yellow fever deaths in coming months from the current outbreak in Angola and Congo, but it will be "incremental not exponential."

World Health Organization emergencies chief Bruce Aylward announced a vast expansion of a vaccination campaign in the two African countries to eventually reach 30 million people. Some 14.5 million have already been vaccinated since the outbreak began earlier this year.

WHO on Thursday reported 355 "confirmed, probable and suspected" yellow fever deaths in Angola, and 75 in Congo.

Aylward said the latest outbreak of a virus for which a vaccine has existed since the 1930s is presenting new challenges, mainly in urban areas.

WHO will also provide fractional doses to help make limited vaccine stocks go further.