The global swine flu caseload surged past the 10,000 mark on Wednesday with the crisis escalating in Asia despite stringent efforts to contain the virus.
The number of confirmed swine flu cases now stands at 10,243 in 40 different countries, said a spokeswoman for the World Health Organisation. The number of dead stands at 80, spokeswoman Fadela Chaib added.
"There is an increase of 413 cases in the past 24 hours," she said.
Much of the increase came from the United States, where authorities raised their number of cases by 346 to 5,469 cases in 47 states plus Washington's District of Columbia.
The number of confirmed A(H1N1) infections in Mexico, the epicentre of the outbreak, rose by almost 100 in 24 hours to 3,660, health authorities said as they also announced four new deaths. Local officials say 76 people have died in Mexico, although not all of these deaths have been registered by the WHO.
And as dozens more cases were reported in Japan, Taiwan became the latest Asian government to record a case of the virus on its soil -- in an Australian doctor who arrived by plane from Hong Kong earlier in the week.
Meanwhile, Australia itself reported two new cases, including a boy aged nine, raising its overall number of confirmed infections to three.
The boy had returned to Melbourne city with his family on a flight from Los Angeles on May 12, and began showing flu-like symptoms this week, Victoria state health officials said.
The mounting crisis has overshadowed proceedings at the WHO's ongoing annual assembly where UN chief Ban Ki-moon urged people on Tuesday to remain vigilant as previous pandemics had shown flu outbreaks could start mild and worsen.
"That is why the world must remain vigilant and alert to the warning signs," Ban said.
The WHO has already raised its alert level to five out of a sliding scale of six, indicating that a full-fledged pandemic is imminent.
The top level would indicate sustained community transmission in a second region outside the Americas and the escalating number of cases in Asia has increased the prospects of the red alert bell being sounded.
The outbreak spread on Wednesday to a third western prefecture in Japan and experts warned it may also have reached Tokyo, which with almost 36 million people is the world's most populous urban area.
Face masks have become ubiquitous on buses, commuter trains and in shopping centres of affected areas in Japan where 228 people have been infected.
Many of the cases have been among school students, prompting authorities to close more than 4,400 schools, colleges and kindergartens for the rest of the week to slow the spread of the virus.
The western commercial hub of Kobe remained the worst affected, announcing more than 20 new infections among roughly three dozen new cases confirmed across Japan Wednesday morning.
Governments in Asia, where memories of the 2003 bird flu crisis remain raw, have been swift to quarantine both locals and foreigners in a bid to stop swine flu in its tracks.
But a group of some 20 foreign tourists held in Tibet over fears an Italian woman with them had swine flu were released Wednesday after tests showed she was suffering from common influenza, Chinese officials said.
The 42-year-old Italian was quarantined in a hospital in Zhangmu town, near the border with Nepal, after crossing the border into China on Saturday with 23 other members of her tour group while suffering from a fever.
"A(H1N1) flu was excluded for the Italian tourist on the morning of May 20, she was diagnosed with seasonal flu, and the 23 other tourists have been released from quarantine," the health ministry said in a statement.