The World Health Organization acknowledged Thursday what many health experts have been saying for weeks: The outbreak of the H1N1 virus is now a pandemic.

"The world is now at the start of the 2009 influenza pandemic," said Dr. Margaret Chan, director-general of the WHO, at a Geneva teleconference. "This virus is now unstoppable."

In a letter sent to member countries, Chan said she is officially raising the agency's infectious disease alert to Phase 6, its highest level, in recognition of the fact that the virus is undergoing community-wide transmission in Australia as well as in North America.

Such spread in two distinct regions of the world is the primary criterion for raising the alert level.

But the agency also said the pandemic is only "moderate in severity" and cautioned against overreactions to the increased alert level.

The announcement marks the first global influenza epidemic in 41 years. The last one was the Hong Kong flu epidemic of 1968, which killed an estimated 1 million people worldwide.

So far, the H1N1 flu pandemic has accounted for 28,774 laboratory-confirmed cases and 144 deaths in 74 countries, although health officials think many times that number have been infected but have not been tested because those peoples' illnesses have been mild.

A normal seasonal flu outbreak kills about 250,000 to 500,000 people worldwide.

In most industrialized countries, the rise in the alert level will have little practical effect because

Health authorities already were behaving as though a pandemic had been declared. In the United States, there have been more than 13,000 cases, more than 1,000 hospitalizations and at least 27 deaths.

"We have been reacting as though it were a pandemic already," said Dr. Anne Schuchat, director of the Centers for Disease Control and Prevention's National Center for Immunization and Respiratory Diseases.

But it will accelerate the production of a vaccine against the new virus. Several nations have signed contracts with manufacturers that call for vaccine production if a pandemic is declared.

Most of the manufacturers have received "seed stock viruses" from the CDC in the past two weeks, allowing them to begin the lengthy process of growing the virus in eggs and producing vaccines.

But vaccines will not be available until September at the earliest, and the supply will be limited.

The announcement will have more impact on Third World countries, freeing up additional funds for treatment and prevention and helping make stocks of antiviral drugs more readily available.