April 29 (Bloomberg) -- The World Health Organization said the first pandemic since 1968 is imminent after an outbreak of swine flu spread to eight nations and 11 U.S. states.

The WHO’s Director-General Margaret Chan raised the agency’s six-phase pandemic alert to 5 from 4, the second jump in three days, at a news conference in Geneva today. Level 5 means a pandemic, or a global outbreak of a new virus, is near.

“The biggest question right now is this: how severe will the pandemic be,” Chan said. World nations “should treat this as an opportunity to ramp up preparedness.”

WHO and other health authorities have begun developing a vaccine against the disease, linked to 159 deaths in Mexico and 1 in the U.S. More than 100 swine-flu cases have been confirmed in North America, Europe and the Pacific, and the virus will probably spread, Keiji Fukuda, WHO’s assistant director-general for health security and environment, said today in a conference call with reporters.

“We have been preparing all along as if this is going to stage 6,” Janet Napolitano, U.S. Homeland Security secretary, said at a news conference in Washington earlier today. “Our preparations are for a situation in which this does become a full-fledged pandemic.”

To contact the reporters on this story: Naomi Kresge in Geneva at nkresge@bloomberg.net; To contact the reporter on this story: Dermot Doherty in Geneva at ddoherty9@bloomberg.net

Last Updated: April 29, 2009 16:18 EDT