"At this time, the global assessment is that we are seeing a moderate pandemic," WHO Director-General Dr. Margaret Chan told reporters on a teleconference.
Raising the level to phase 6 means the U.N. health body is declaring a pandemic, the first since global flu pandemic in more than four decades. Moving to phase 6, the highest level, means a pandemic has begun. It will trigger drugmakers to speed up production of a swine flu vaccine and prompt governments to devote more money to containing the virus.
The last pandemic — the Hong Kong flu of 1968 — killed about 1 million people. Ordinary flu kills about 250,000 to 500,000 people each year.
The move will trigger heightened health measures in the WHO's 193 member states as authorities brace for the worldwide spread of the virus that has so far caused mainly mild illness. But acting on the recommendation of flu experts, the WHO reiterated its advice not to close borders or impose travel restrictions to halt the movement of people, goods and services.
A move to phase 6 reflects the fact that the disease, widely known as swine flu, was spreading geographically, but not necessarily indicate how virulent it is.
"Phase 6, if we call a phase 6, doesn't mean anything concerning severity, it is concerning geographic spread ... Pandemic means global, but it doesn't have any connotation of severity or mildness," WHO spokesman Gregory Hartl told Reuters.
"In fact, what we are seeing with this virus so far is overwhelmingly to date mild disease. So we would think that this event is really a moderate event for the time being, because the numbers are high but the disease is overwhelmingly mild," he told Reuters.
On Wednesday, the WHO reported that the virus has infected at least 27,737 people in 74 countries and caused 141 deaths. Most cases have been in North America, but Europe and Australia have seen a sharp increase in recent days.