The United Nations agency, which reported four cases around the Red Sea port city of al-Hudaydah last week, said 18 more children had contracted the disease in the poor Arab state.
It is the latest setback to the WHO's campaign to wipe out the transmission of polio worldwide by the end of the year. An epidemic, which originated in Nigeria, has swept across Africa since mid-2003.
"What we are facing now is a major epidemic of polio in Yemen," David Heymann, head of the WHO's polio eradication programme, said in Geneva on Friday.
It is not yet known whether there have been any fatalities among the victims, who include children from all over Yemen.
"Experts fear the number of cases will rise in the immediate future," the WHO said. It said it was investigating other suspected cases, and low immunisation rates among Yemeni children could facilitate the outbreak's spread.
"We'll probably never be able to know whether the virus came from pilgrimage (to Makka) or from guest workers. What is important is that the virus is there, and now we have to stop it"
WHO polio eradication programme
The polio virus mainly affects children under the age of five and can cause irreversible paralysis in a matter of hours.
Yemen, which last reported polio in 1996, is the 15th previously polio-free country to be re-infected since mid-2003, including 13 in Africa and Saudi Arabia.
The WHO was awaiting genetic analysis of the virus to determine whether it had come from neighbouring Saudi Arabia or Sudan, which have both registered cases in the past year.
"We'll probably never be able to know whether the virus came from pilgrimage (to Makka) or from guest workers. What is important is that the virus is there, and now we have to stop it," Heymann said.
Yemen conducted a mass vaccination campaign in mid-April and further immunisation rounds are planned in May and June.
A monovalent oral vaccine against type 1 polio, recently licensed by Sanofi-Pasteur, a unit of French drugmaker Sanofi-Aventisis, is being used in Yemen.
Nigeria's Kano state officials
banned polio vaccines in 2003
"We are very confident this vaccine will help us finish polio," Heymann said.
In 2004, polio cases worldwide jumped to 1267, up from 784 the previous year, the WHO said.
WHO's eradication campaign suffered a severe blow in 2003, when Nigeria's Kano state banned vaccines, because local leaders said they were part of a Western plot to spread HIV and infertility.
Immunisation resumed last July, but the 10-month ban helped the virus reach epidemic proportions on the African continent.