The World Health Organization says more than 1,000 cases of the swine influenza A-H1N1 have been confirmed in 20 countries.
WHO Director-General Margaret Chan reported the latest case count as she briefed the U.N. General Assembly Monday via video link from Geneva.
Despite the rise in confirmed flu cases worldwide, authorities in Mexico, the epicenter of the outbreak, say the virus is in decline there. Restaurants and other businesses in Mexico that had been closed in an attempt to slow the spread of the flu virus will reopen Wednesday.
The virus is being identified in more cases daily in the United States, elsewhere in the Americas, Europe and Asia. The United States is reporting 286 confirmed cases in 36 states.
In a newspaper interview, the WHO chief says it is too early to conclude the swine-flu danger is passing. Chan warns the new virus could return with renewed intensity and cause more deaths in the months ahead.
Mexico has reported 25 confirmed deaths from the new virus, while the United States has one.
WHO raised its pandemic alert level to phase five last week, one step below the declaration of a pandemic. U.N. Secretary-General Ban Ki-moon said Monday there are no immediate plans to raise the alert level to six. He added that such a declaration would be a statement about the geographical spread of the virus, not its severity.
The U.N. health agency says the illness has been confirmed in Austria, Canada, Costa Rica, Colombia, Denmark, El Salvador, France, Germany, Hong Kong, Ireland, Israel, Italy, Netherlands, New Zealand, South Korea, Spain, Switzerland and the United Kingdom. Portugal announced its first confirmed case Monday.
WHO has sent 2.4 million anti-viral treatments to 72 developing countries for use in a possible pandemic.
Mexico's Health Minister Jose Angel Cordova said Sunday the outbreak appeared to peak in Mexico between April 23 and April 28, and that the number of severe cases has declined since then.
Scientists have yet to determine the strength and deadliness of the previously unknown flu strain, which is a mix of swine, avian and human viruses. The new strain has raised concerns because it is spreading outside the normal flu season in the Northern Hemisphere, and has been deadly among young, healthy adults.
Health experts are concerned the flu could reappear and become a more serious threat near the end of this year - the winter flu season in the Northern Hemisphere - when influenza viruses typically cause widespread respiratory illness and many deaths.
Some information for this report was provided by AP and Reuters.