The World Health Organisation on Wednesday raised its flu alert to phase five out of six, WHO chief Margaret Chan said, signalling that a pandemic was "imminent" following the swine flu outbreak.

"I have decided to raise the influenza pandemic alert from phase four to phase five," the WHO director general announced after an emergency meeting of the UN health agency's pandemic experts.

"This is a signal to governments, ministries of health... to the pharmaceutical industry, that certain actions now should be undertaken with increased urgency," she told journalists.

"Preparedness measures undertaken because of the H5N1 influenza was an investment and we are now benefiting from this investment," she said.

She underlined that the threat following the swine flu outbreak focused on Mexico and the United States "must be taken seriously".

Phase five, one step short of a full pandemic, is characterised as a "strong signal that a pandemic is imminent and that the time to finalise... the planned mitigation measures is short," according to the WHO's global emergency planning.

It also acknowledge sustained human-to-human spreading in at least two countries in a WHO region, in this case Mexico and the United States, Chan told journalists.

The move came after a meeting of scientific experts on the emergency flu pandemic panel recommended raising the alert level from four.

The US government is prepared to cope with a "full-fledged pandemic" of swine flu if necessary, but the public should not be alarmed, Homeland Security Secretary Janet Napolitano said on Wednesday.

"We have been preparing all along as if this is going to be stage six," she told reporters.

"Our efforts have been to stay ahead of whatever number WHO assigns. And therefore, our preparations are for a situation in which this does become a full-fledged pandemic," Napolitano said.

"We are preparing for the worst, hoping for the best," she said, after a Mexican toddler became the first person to die in the United States of H1N1 swine flu and the number of infections rose to at least 91.

But Napolitano, again rejecting calls from US lawmakers to close the nation's borders, said "panicking" was unhelpful and underlined that a regular flu season claims about 36,000 American lives every year.

话说他这等级是根据什么定的  一直以为是病毒危害程度  现在看貌似是范围

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