The number of polio cases globally has reached 333 so far this year, almost double the number for the same period last year. Total cases last year reached 783.
In Nigeria, where, 257 cases have been reported this year.
"There is no question that the virus is spreading at an alarming pace," said Dr. David Heymann, who is overseeing the U.N. health agency's effort to eradicate the disease.
Polio is a water-borne disease that usually infects young children, attacking the nervous system and causing paralysis, muscular atrophy, deformation and sometimes death.
Nearly 1,000 children in 125 countries were being infected daily by polio in 1988 when WHO and other health organizations launched the Global Polio Eradication Initiative.
That number dropped to 483 for the whole of 2001, and health officials declared the disease was eradicated in Europe, the Americas, much of Asia and Australia.
Epidemiologists now fear a major epidemic this fall — the start of the polio "high season." The disease has appeared already in nine sub-Saharan African countries, after being limited to only two at the beginning of last year.
"Right now we're at the end of the low season of polio," Bruce Aylward, who coordinates the polio initiative at WHO, told reporters.
"This is right when polio should be at its lowest level. It suggests that the virus is gathering steam to come roaring out," he said.
WHO confirmed that a child was paralyzed by polio on May 20 in Darfur, the first case in Sudan in more than three years.
Health experts have long warned of epidemics in Darfur, where thousands have been killed and more than 1 million left homeless in a 15-month conflict between government-backed Arab militias and the black African Muslim population.
The battle against the disease has stalled in the face of resistance to immunization programs in Nigeria's heavily Muslim Kano state.
Some Islamic leaders claimed that the vaccines were part of a U.S.-led plot to spread infertility and AIDS among African Muslims. Nigerian federal officials and the United Nations deny the claims.
The disease has also been found in previously polio-free Benin, Burkina Faso, Central African Republic, Chad and Ivory Coast. Botswana reported its first new infection in February.
Four other countries — Egypt, Pakistan, Afghanistan and India — are on schedule to eradicate the disease before the end of the year, WHO said.
The U.N. agency said it urgently needs $25 million to launch a vaccination campaign across 22 African nations in October. Officials said they could meet a Jan. 1 deadline to eradicate the disease if they succeed in immunizing 74 million children this fall.
By Jonathan Fowler