The number of cases will probably soon exceed 100, as many more suspected cases are still being investigated, said Oliver Rosenbauer, a spokesman for the World Health Organization's polio eradication campaign.

Before the first polio cases in the Middle Eastern country surfaced last month, Yemen was believed to be to be free of the disease.

"They are having a pretty big epidemic there," Rosenbauer said. "But we should be able to stop the virus relatively quickly."

WHO has organized a national immunization drive in Yemen to take place in late May, for which 6 million vaccination doses have already been sent. The doses are expected to arrive in Yemen early next week, Rosenbauer said.

Yemen is one of 16 previously polio-free countries that have reported new cases since 2003 after a vaccine boycott in Nigeria was blamed for causing an outbreak that spread the disease to other countries.

Hard-line Islamic clerics in northern Nigeria led the immunization boycott, claiming the polio vaccine was part of a U.S.-led plot to render Nigeria's Muslims infertile or infect them with AIDS. Vaccination programs restarted in Nigeria in July 2004 after local officials ended the 11-month boycott.

Last year, some 1,267 people were infected in the world -- with 792 of those in Nigeria. The total new cases in 2005 stands at 196, according to WHO, with Nigeria and Yemen listed as the worst-affected countries.

Health officials in Yemen have given contradictory reasons over the spread of the disease, with some saying the epidemic spread to the country from Nigeria, and others saying expired vaccines caused the outbreak.

Rosenbauer said vaccinations can sometimes cause polio-like symptoms but would not be responsible for the exportation of the "wild polio virus" which has been identified in Yemen.

The outbreak in Yemen may be stopped by a targeted intervention from the health agency, but it could be devastating if the virus crosses the Red Sea to Somalia, because lack of security will make it very difficult to conduct a vaccination campaign, he said.

"We have already seen polio reintroduced in Ethiopia," Rosenbauer said. "If it spreads to Somalia it will be a problem because it is logistically very, very challenging there."

Somalia has been without a central government since clan-based warlords overthrew the dictator Mohamed Siad Barre in 1991. Warlords then turned on each other, plunging the Horn of Africa nation of 7 million into anarchy.

Polio is a waterborne disease that usually infects young children, attacking the nervous system and causing paralysis, muscular atrophy, deformation and sometimes death.

When WHO launched its anti-polio campaign in 1988, the worldwide case count was more than 350,000 annually.