"I'm not predicting the pandemic will blow up, but if I miss it and we don't prepare, I fail. I'd rather over-prepare than not prepare".
She indicated that the WHO was likely to raise its flu alert level to the top of its six-point scale - meaning the virus had spread between different regions and a global pandemic was under way.
But she stressed that this did not necessarily mean every country would be affected or that there would be many more deaths.
"It is a signal to public health authorities to take appropriate measures," she said.
The WHO said on Monday that 985 cases of the virus had been officially reported across 20 countries.
Person-to-person transmission has been confirmed in six countries.
Slightly more than 100 people in Mexico are thought to have died from the swine flu strain, although only 25 cases have been confirmed.
Elsewhere:
• Portugal's health ministry confirmed its first case of swine flu
• The number of confirmed cases in Italy rose from two to four
• Spain raised the number of confirmed cases from 40 to 54, making it the worst-hit country in Europe
• Turkish media reports of a patient dying from swine flu in the southern Antalya province were denied by a hospital spokeswoman to Reuters news agency
'Not over yet'
In the US, the number of confirmed cases rose from 160 to 226. Officials said this was because the results of lab tests were now coming through, rather than because of a new surge in cases.
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Mexicans trying to get back to normal
But an expert from America's Centers for Disease Control and Prevention said the virus was fairly widespread.
"Virtually all of the United States probably has this virus circulating now," Dr Anne Schuchat said.
"That doesn't mean that everybody's infected but within the communities, the virus has arrived."
She said that although she expected cases to become more severe and to lead to deaths, this in itself would not be unusual as every year 36,000 people died in the US after contracting seasonal flu.
But she warned: "I don't think we're out of the woods yet."
'Discriminatory measures'
Mexican officials are due to decide on Monday whether to allow schools and businesses to reopen after reporting that its most lethal outbreak might be levelling off.
SYMPTOMS - WHAT TO DO
Swine flu symptoms are similar to those produced by ordinary seasonal flu - fever, cough, sore throat, body aches, chills and fatigue
If you have flu symptoms and recently visited affected areas of Mexico, you should seek medical advice
If you suspect you are infected, you should stay at home and take advice by telephone initially, in order to minimise the risk of infection
Health Minister Jose Angel Cordova said that the virus appeared to have peaked between 23-28 April.
"The evolution of the epidemic is now in its declining phase," he told a news conference.
President Felipe Calderon, meanwhile, denounced what he described as "discriminatory measures" against Mexicans by some countries "because of ignorance and misinformation".
He did not name any countries but his remarks came after Mexico criticised China for placing in quarantine up to 70 Mexicans, even though they had shown no sign of having contracted swine flu.
Mexico said it was planning to send a plane to Beijing to pick up the Mexicans.
China denied it was being discriminatory. "This is purely a question of health inspection and quarantine," the foreign ministry said.
More than 300 guests at a hotel in Hong Kong have also been under quarantine for a week after a fellow guest contracted the disease.
One guest, Kevin Ireland, spoke of a "sense of paranoia" initially by the authorities - who tackled the Sars outbreak in 2003 - but added that "there's no restriction on any kind of interaction" among the guests.
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