As polio cases surge in Nigeria and the virus spreads to other countries, West and Central Africa are on the brink of the largest polio epidemic in recent years, alarmed officials of the World Health Organization and UNICEF said Tuesday.

The 60 cases just reported by Nigeria for the final week in April, the latest period for which information is available, constitute the largest weekly number for any country in recent years, the officials said.

Last weekend, genetic testing of a child who became paralyzed on May 20 confirmed that a 10th country, Sudan, had polio that had come from Nigeria. The WHO, a U.N. agency based in Geneva, called the new developments "a stark warning."

Even though caseloads outside Nigeria are relatively small, health officials say they may greatly understate polio's spread and the danger of an epidemic.

Only about 1 in 200 people infected by the virus develops paralysis; an overwhelming majority experience only diarrhea and other nonspecific gastrointestinal symptoms. Yet any infected individual can spread the disease.

That signals the potential for a large epidemic as polio enters the season when transmission is highest, the officials say. The polio virus spreads through feces, and contamination occurs more often when sewage backs up during the rainy season in Africa and Asia.

The threat comes just as the World Health Organization and its partners expected that their $3.1 billion program was about to make polio the second disease to be conquered around the world, after smallpox. Instead, Dr. Bruce Aylward, a polio official at the agency, said, "we could see thousands paralyzed at a time when the disease should be eradicated."

Health officials and other experts attribute the current outbreak in Nigeria and the virus' spread to other countries to a decision by the government of the northern state of Kano last August to stop immunizing children. Political and religious leaders there claimed that the vaccine made girls infertile.

Allegations about impurities in the vaccine have caused alarm and confusion among many people in Nigeria and elsewhere. Tests of the vaccine have refuted such claims, said Dr. David L. Heymann, the director of the World Health Organization's polio eradication program. The vaccine purchased by UNICEF meets the organization's standards and has been used safely to eliminate the disease from many other countries.

In May, Kano's governor, Ibrahim Shekarau, pledged to resume vaccinations. But that has not happened, Heymann said. Meanwhile, polio has spread not only in Kano but from that state to other parts of Nigeria.