The announcement comes during a mainly upbeat assessment of progress in a global vaccination campaign.
A 15-month-old girl in the Somali capital Mogadishu was confirmed to be infected by the paralysing disease on Monday, the head of the United Nations' health agency's campaign against polio, Dr Bruce Aylward, said in Geneva.
The WHO has not determined how the infant had caught polio, but suspects it might have come from Yemen, across the Gulf of Aden, where 400 polio cases have been reported, Aylward said.
"With Somalia, 19 countries have been reinfected in the last 24 months," he said.
On Tuesday, the WHO launched a vaccination programme in Yemen as well as in four Horn of Africa countries - Djibouti, Eritrea, Ethiopia and Somalia.
Health staff also began a vaccination campaign in Sudan and some parts of Kenya in East Africa and of the vast Democratic Republic of Congo (DRC), straddling the centre of the continent.
Vaccinators conduct a door-to-
door immunisation campaign
Of 17 African countries, polio is believed to have been transmitted by various means from Nigeria, where an 18-month suspension of a vaccination programme led to a spread of the disease at a time when UN health officials had hoped it was finally all but wiped out in Africa.
The resurgence of polio - attributed to a delay in vaccinations because influential Muslim leaders in several northern Nigeria states initially objected on religious and political grounds - has been an immense setback to a UN programme to eliminate the disease worldwide by the end of 2005.
Somalia, a nation of about 10 million people, has been carved up among rival warlords and torn apart by factional warfare since the overthrow in 1991 of president Mohammed Siad Barre.
Polio has resurfaced in parts of
Asia as well, including Indonesia
The other country listed in Africa was Angola, but reinfection in that southwestern nation was believed to have come from India.
Aylward was optimistic about a "good prospect overall" for stopping an epidemic in the Horn of Africa before the end of the year after it is eliminated again in countries bordering on and close to the west African giant, Nigeria.
"The risk is Nigeria," he said, estimating that it could take a year to eradicate the disease there once more.
Polio has also resurfaced in the Southeast Asian nation of Indonesia after a 10-year absence, Aylward said, but the number of cases there "is dropping fast".
Apart from countries that have been reinfected, the disease remains endemic in six states: Nigeria, its northern neighbour Niger, Egypt, India, Pakistan and Afghanistan, according to the WHO.
Previous vaccination campaigns had been so successful, reaching two billion children, that the number of cases registered worldwide fell from about 350,000 in 1998 to fewer than 800 in 2003.
Last year, the figure went back up to almost 1300.