"Influenza pandemics must be taken seriously precisely because of their capacity to spread rapidly to every country in the world," she told a news conference in Geneva.
"The international community should treat this as a window of opportunity to ramp up preparedness and response.
"Above all this is an opportunity for global solidarity as we look for responses and solutions that benefit all countries and all of humanity."
Hugh Pennington, professor of bacteriology at the University of Aberdeen, Scotland, told Al Jazeera a pandemic may not mean vast numbers of deaths.
"I don't think we should lose sight of the fact that a pandemic might be quite benign. Even if it spreads round the world it may not be any nastier than the ordinary flu we see every year,” he said.
The health organisation's increase in alert level means there will be greater efforts to produce a vaccine.
Paul Garwood, a WHO spokesman in Geneva, told Al Jazeera that while there was no vaccine for the new virus, it could be treated with drugs.
"It's new, it's causing severe illnesses in Mexico and people should be aware of this.
"At the same time we know that this illness can be treated with drugs that are on the market and that symptoms are mild and they are recovering."
More countries confirm cases
The WHO has so far confirmed human cases of swine flu in Mexico, the US, Canada, Britain, Israel, New Zealand and Spain. Separately, Germany, Austria, Switzerland and the Netherlands have also confirmed cases.
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The virus is believed to have claimed up to 160 lives as of Wednesday, with the Mexican government confirming eight deaths directly linked to the flu, downgrading its previous figure of 26.
In the US, a 23-month-old Mexican child on Wednesday became the first person outside of Mexico to die from swine flu.
The family of the child had recently travelled from Mexico City, crossing the border into Texas, US officials said.
At least five other people across the US were also taken to hospital after falling ill from the virus.
Barack Obama, the US president, described the situation as serious and urged US schools with suspected cases to close.
In a speech marking his first 100 days in office, he also urged Americans to take precautions such as washing hands and avoiding work, school and travel if ill.
"We are continuing to closely monitor the emerging cases of the H1N1 flu virus throughout the United States … this is obviously a very serious situation, and every American should know that their entire government is taking the utmost precautions and preparations," he said.
Obama called on local officials to be vigilant in tracking suspected cases and said "more extensive steps" may be required.
He has asked congress for $1.5bn so the government can respond adequately to the threat.
Al Jazeera's Franc Contreras, reporting from Mexico City, the capital of the country where the virus is thought to have originated, said the government downgraded the number of deaths linked to the flu from 26 to eight on Wednesday.
Eight confirmed in Mexico and 159 more suspected. One death in the United States
Countries with confirmed cases: Mexico, the US, Canada, New Zealand, Spain, Israel, Britain, Germany, Austria, and Switzerland
Countries with suspected cases: Australia, Brazil, France, Chile, Denmark, Colombia, Germany, Norway, South Korea, Guatemala
Annual influenza epidemics are thought to result in three to five million cases of severe illness and between 250,000 and 500,000 deaths around the world, WHO says.
Mexico's president responded to the raised alert by telling citizens to stay home with their families from Friday, May 1, for a five-day partial shutdown of the economy.
In his first televised address since the crisis erupted last week, Felipe Calderon said on Thursday that the country will suspend non-essential work and services, including some government ministries, for the five-day period.
"There is no safer place than your own home to avoid being infected with the flu virus," Calderon said.
Jose Angel Cordova, the health minister, said the food, medical and transportation sectors would not be affected.
But the virus has begun to bite hard in an economy already reeling from the global downturn.
Cuba has banned all flights to and from its neighbour and Argentina announced on Tuesday a five-day ban on flights arriving from Mexico.
Mexico has estimated the outbreak is costing companies at least $57m a day and contributing to a 36 per cent loss in tourism revenue.
The country's finance ministry has established a $450m fund to fight the virus, after health authorities said up to 159 people there may have died from the new strain of H1N1 virus.
Health ministers from the region are to hold an emergency meeting on Saturday to discuss the threat of a pandemic while WHO was set to hold a "scientific review" of the outbreak to collect what was known about how the disease spreads, how it affects human health and how it can be treated.