Washington - US President Barack Obama said late Wednesday that there were no plans to close the country's border with Mexico despite a growing swine flu outbreak, but assured the public that his government was doing all it could to contain the threat.

Obama said closing the border was "akin to closing the barn door after the horses are out," as there were already nearly 100 cases confirmed in the United States, including one death.

"This is a cause for deep concern, but not panic," Obama said in a news conference at the White House. "The key now, I think, is to make sure that we are maintaining great vigilance."

The World Health Organization upgraded its alert level on the swine flu outbreak to phase five, one below its highest and meaning that a pandemic is imminent, but not inevitable.

The United States confirmed the first death from swine flu on Wednesday - a 23-month-old toddler in Houston, Texas, who had entered the country from Mexico earlier this month.

Dr Richard Besser, acting director of the US Centres for Disease Control and Prevention, said the total number of cases in the US had increased to 91 from 64 recorded on Tuesday.

Ten US states were exposed to the virus, he said. The total included 51 cases in New York, 16 in Texas, 14 in California, 2 each in Massachusetts and Michigan; and 1 each in Arizona, Nevada, Indiana, Kansas and Ohio.

Secretary of Health and Human Services Kathleen Sebelius said the US had stockpiled 50 million treatment portions of two anti-viral medicines, Tamiflu and Relenza, and had started shipping them to states with confirmed cases.

"These drugs are effective in treating patients who have acquired the 2009 H1N1 virus," she said. "We are likely to see additional deaths from the outbreak."

Marine Commandant General James Conway said a US Marine was infected with swine flu and he and his roommate were confined to their quarters at their base in California.

Another 30 soldiers at the Twentynine Palms base who had contact with the ill Marine have been placed on restricted duty.

In New York, officials Wednesday downplayed the fact that the state has the highest total of confirmed cases of swine flu in the US, saying the situation was inexplicable - even as health workers were investigating 75 more probable cases and some schools remained closed.

New York Governor David Paterson and his health commissioner, Richard Daines, who noted there was no vaccine for swine flu, said they did not consider the swine flu outbreak any more important than unseasonal flu in springtime.

"The irony of this is that the flu does not seem to affect Americans the same way it affects Mexicans," Paterson said.

Officials in Mexico, the epicentre of the outbreak, on Wednesday night raised the number of confirmed swine flu infections from 49 to 91. Mexican Health Minister Jose Angel Cordova updated the death toll to eight, from seven.

There have been 159 deaths and 2,498 infections in Mexico's flu outbreak. Of these, 1,311 people remain in hospital and most have not yet been identified as swine flu cases. (dpa)