The World Health Organization on Thursday acknowledged what many health experts have been saying for weeks: The outbreak of 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 now undergoing communitywide 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 that the pandemic is only “moderate in severity” and cautioned against overreactions to the increased alert level.

The announcement marks the advent of 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, also called swine flu, pandemic in 2009 has accounted for 28,774 laboratory-confirmed cases and 144 deaths in 74 countries, although health officials believe many times that number have been infected but have not been tested because those cases 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 were already behaving as though a pandemic had been declared.

“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 Disease.