The number of cases in Yemen will probably soon exceed 100, as many more suspected cases are being investigated, said Oliver Rosenbauer, a spokesman for the World Health Organisation's polio eradication campaign.
"They are having a pretty big epidemic there," Rosenbauer said. "But we should be able to stop the virus relatively quickly."
Meanwhile, in Indonesia, which like Yemen was thought to be polio-free, two more cases have been confirmed, the WHO said. Indonesia, which reported its first case on 3 May, now has six.
The WHO has organised a national immunisation drive in Yemen in late May, for which six million vaccination doses have been sent. The doses are expected to arrive early next week, Rosenbauer said.
Yemen and Indonesia are the latest 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. Yemen's outbreak was first reported on 22 April.
Some Muslim clerics in northern Nigeria led the immunisation boycott, claiming the polio vaccine was part of a US-led plot to render Nigeria's Muslims infertile or infect them with Aids. Vaccination programmes restarted in Nigeria in July 2004 after local officials ended the 11-month boycott.
Spread of virus
The virus has since spread to Benin, Chad, Cameroon, Botswana, Burkina Faso, Central African Republic, Ethiopia, Ghana, Guinea, Mali, Saudi Arabia, Sudan and Togo.
Last year, 1267 people were infected in the world - 792 of those in Nigeria. The total new cases in 2005 stands at 198, according to WHO, with Nigeria and Yemen listed as the worst-affected countries.
Indonesia has reported two more
cases of polio
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.
Officials in Yemen said parents of infected children are planning lawsuits against the Health Ministry for failing to halt the spread of the disease.
Rosenbauer said vaccinations could 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 WHO's polio eradication programme is focusing primarily on the six countries where polio remains endemic - Nigeria, India, Pakistan, Niger, Afghanistan and Egypt.
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."
Polio is a water-borne disease that usually infects young children, attacking the nervous system and causing paralysis, muscular atrophy, deformation and sometimes death.
When the WHO launched its anti-polio campaign in 1988, the worldwide case count was more than 350,000 annually.