Over 400 new cases of swine flu have been confirmed by the US government as the nation's total number of infections topped 5,000. The government also said it remained on guard for an autumn upsurge.
The Centers for Disease Control and Prevention (CDC) said the number of confirmed and probable cases in 47 states plus Washington's District of Columbia now stood at 5,123, up from 4,714 on Friday.
A teacher at a New York school who had been in critical condition for days died Sunday after being infected with the A(H1N1) virus, taking the US death toll to six. Only Mexico has suffered more deaths from the disease.
The CDC has downgraded a travel warning for Mexico but Anne Schuchat, director of the agency's center for immunization and respiratory diseases, took issue with media reporting that is now playing down the outbreak.
"The H1N1 virus is not going away, despite what you may have heard," she told reporters, stressing that the number of confirmed cases in the United States may be only the "tip of the iceberg."
"Unfortunately based on the trends we're seeing, we do expect more illness, more hospitalizations, and more deaths," Schuchat said, with more than 200 people already requiring hospital treatment in the United States.
The official backed the actions of New York authorities, where 12 schools around the city had been temporarily closed after suspicious outbreaks, forcing 11,000 children to miss classes.
"We cannot stop the virus from spreading. We should not be surprised to see more cases," New York Mayor Michael Bloomberg warned.
City officials said their fight against swine flu would focus on those weakened by pre-existing conditions, as was the case in the weekend death of the 55-year-old assistant principal at a school in the Queens neighborhood.
The US update followed a sharp increase in swine flu infections in Japan, which stoked fears that the virus may have taken hold in another continent beyond its source in North America.
The World Health Organization has warned that sustained transmission in a country in another region, without a direct link to travelers, would warrant the declaration of a global pandemic.
WHO chief Margaret Chan warned on Monday that the world might be facing the calm before a swine flu storm, as she faced pressure from Britain and other nations not to rush into declaring a pandemic.
Schuchat said she did not know if US officials attending the WHO's annual assembly in Geneva had sided with Britain in urging Chan to desist from a pandemic declaration.
But she said that such a declaration would add nothing to the "aggressive" efforts of US authorities to contain the outbreak.
"Things could change quickly and we do continue our efforts to prepare, particularly for the fall, where history tells us we might have a second wave of this virus," the CDC official said.