Margaret Chan, the WHO's director-general, said all countries should "activate their pandemic preparedness plans," adding that "it really is all of humanity that is under threat in a pandemic".
The health body also announced that it had changed the name of the flu to 'influenza A (H1N1)' and said it would stop using the term swine flu to avoid confusion over the danger posed by pigs.
The decision came a day after Egypt began slaughtering thousands of pigs in a misguided effort to prevent the flu.
"Influenza pandemics must be taken seriously precisely because of their capacity to spread rapidly to every country in the world," she said.
Keiji Fukuda, the WHO's deputy chief, said Roche, the Swiss drugmaker, had indicated it was stepping up production of Tamiflu to deal with the virus.
At a press conference in Geneva on Thursday, he also said there was nothing to suggest raising the alert level to phase six.
The International Federation of Red Cross and Red Crescent Societies has said it could deploy hundreds of thousands of volunteers across the world in response to H1N1.
Launching an appeal on Thursday for $4.4m to help tackle the crisis, a spokesman said it would be "a few simple community-placed behaviours that will play a major role in preventing the disease further".
Hugh Pennington, a professor of bacteriology at the University of Aberdeen, Scotland, told Al Jazeera: "I don't think we should lose sight of the fact that a pandemic might be quite benign.
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"Even if it spreads round the world it may not be any nastier than the ordinary flu we see every year," he said.
The WHO has also pointed out that most of the previously reported flu cases had "recovered from the disease without requiring medical attention and without antiviral medicines".
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.
"We know that this illness can be treated with drugs that are on the market and that symptoms are mild and [patients] are recovering," he said.
The virus is believed to have claimed up to around 160 lives, with the Mexican government confirming 12 deaths directly linked to the flu, downgrading its previous figure of 26.
In the US, the Center for Disease Control and Prevention confirmed the national total of H1N1 cases had risen to 109 on Thursday, with the majority in New York.
On Wednesday, a 23-month-old Mexican child in Texas became the first person outside of Mexico to die from the virus.
Barack Obama, the US president, has asked congress for $1.5bn so the government can respond adequately to the threat.
He urged Americans to take precautions such as washing hands and avoiding work, school and travel if ill.
"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.
Twelve confirmed in Mexico out of 159 suspected. One death in the United States
Countries with confirmed cases: Mexico, the US, Canada, New Zealand, Spain, Israel, Britain, Germany, Austria, Switzerland, Netherlands
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 has told 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.