The World Health Organization said Wednesday it has confirmed the first cases of yellow fever in an urban area of Latin America in six decades.

Dr. William Perea, yellow fever chief for the U.N. health agency, said the mosquito-born disease can spread particularly fast in suburbs and cities and warned that vaccinations are needed to stem the outbreak.

In crowded urban areas, yellow fever can "spread like a fire in the forest," Perea said, adding that mosquitoes thrive in built-up areas with poor hygiene and sanitation.

WHO officials said there have been nine confirmed cases in the suburbs of Paraguay's capital, Asuncion. The agency said three people had died, though Paraguayan authorities put the death toll at eight.

Perea said in a telephone news conference that many people in Latin America's cities have not been exposed to the virus, so have no immunity. In the jungle, where most yellow fever outbreaks occur, people have better defenses against the virus.

WHO experts said a mass vaccination campaign was under way in Paraguay and was closely monitoring vaccine supplies. Dr. Marlo Libel, of WHO's regional office for the Americas, said the situation was "under control."

The yellow fever outbreak is Paraguay's first since 1974. The last yellow fever cases in any Latin American city were in the 1940s in Brazil, Libel said.

An estimated 30,000 people worldwide die annually from the disease, WHO says. Symptoms can include fevers, vomiting, jaundice and bleeding from the mouth, nose, eyes and stomach.