Mexico health department spokesman Carlos Olmos confirmed the move by the WHO to raise the alert level from Phase 3 to Phase 4.
Putting an alert at Phases 4 or 5 signals that the swine flu virus is becoming increasingly adept at spreading among humans.
That move could lead governments to set trade, travel and other restrictions aimed at limiting the disease's spread.
The WHO's Phase 6 is the pandemic phase, characterized by outbreaks in at least two regions of the world.
U.S. Homeland Security Secretary Janet Napolitano said the United States is preparing as if the swine flu outbreak already is a full pandemic.
The virus was suspected in up to 149 deaths in Mexico, the epicenter of the outbreak with more than 1,600 cases suspected, while 50 cases - none fatal - were confirmed in the United States.
Worldwide there were 79 confirmed cases, including six in Canada, one in Spain and two in Scotland.
The World Health Organization reported a slightly lower figure, 73. The WHO said it was still awaiting official reports from the U.K. about the Scottish cases, and it was reporting different numbers of cases in the U.S. (40) and Mexico (26) from what those governments confirmed.
Russia, Hong Kong and Taiwan said they would quarantine visitors showing symptoms of the virus amid global concern about a possible pandemic.
European and U.S. markets bounced back from early losses as pharmaceutical stocks were lifted by expectations that health authorities will increase stockpiles of anti-viral drugs. The stocks of airlines, hotels and other travel-related companies posted sharper losses.
"Today we've seen increased number of confirmed cases in several countries," WHO spokesman Paul Garwood told The Associated Press. "WHO is very concerned about the number of cases that are appearing, and the fact that more and more cases are appearing in different countries."
President Barack Obama said the threat of spreading infections is cause for concern but "not a cause for alarm."
In Luxembourg, European Union Health Commissioner Androulla Vassiliou urged Europeans to postpone nonessential travel to parts of the United States and Mexico affected by swine flu, toning down earlier comments referring to all of North America.
"I meant a travel advisory, not a travel ban, for travel to Mexico City and those states in the United States where we have outbreaks" of swine flu, he said.
The EU health commissioner only makes recommendations to the 27 member countries; they must make a final decision to set travel advisories through their foreign ministries.
Dr. Richard Besser, acting head of the Centers for Disease Control and Prevention in Atlanta, said the EU recommendation was not warranted. "At this point I would not put a travel restriction or recommendation against coming to the United States."
Spain's first swine flu case - confirmed by the WHO - was a young man in the town of Almansa who recently returned from Mexico for university studies and is responding well to treatment, said Health Minister Trinidad Jimenez. Neither the young man nor any of the 20 other people under observation for the virus were in serious condition.
Scottish Health Secretary Nicola Sturgeon said tests "conclusively" confirmed swine flu in two people also recently returned from Mexico. A government spokeswoman said the two were recovering in Monklands Hospital in the Scottish town of Airdrie with flu-like symptoms. The virus matched the strain of flu that has affected Mexico, said the woman, who spoke on condition of anonymity in line with Scottish government policy.
She said tests were conducted at a Glasgow laboratory before being sent to the Health Protection Agency's Colindale Center for Infections in London, which confirmed the outbreak.
WHO spokesman Peter Cordingley singled out air travel as an easy way the virus could spread, noting that the WHO estimates that up to 500,000 people are on planes at any time.
New Zealand was testing 13 students, their parents and teachers who were showing flu-like symptoms after returning from Mexico, said Health Minister Tony Ryall. Israel, France, Brazil and Switzerland were also conducting tests.
At Germany's bustling Frankfurt Airport, people suspected of having the disease were examined before getting off planes, said the health minister for Hesse state, Juergen Banzer. The policy was in effect since Saturday at continental Europe's second-busiest airport, after Charles de Gaulle in Paris.
Governments in Asia - with potent memories of SARS and avian flu outbreaks - heeded the warning amid global fears of a pandemic.
Singapore, Thailand, Japan, Indonesia, and the Philippines dusted off thermal scanners used in the 2003 SARS crisis and were checking for signs of fever among passengers from North America.
South Korea and Indonesia introduced similar screening.
In Malaysia, health workers in face masks took the temperatures of passengers as they arrived on a flight from Los Angeles.
Russia, Hong Kong and Taiwan said visitors returning from flu-affected areas with fevers would be quarantined.
Australian Health Minister Nicola Roxon said pilots on international flights would be required to file a report noting any flu-like symptoms among passengers before being allowed to land in Australia.
China said anyone experiencing flu-like symptoms within two weeks of arrival had to report to authorities.
India will start screening people arriving from Mexico, the United States, Canada, New Zealand, Spain, Britain and France for flu-like symptoms, said Vineet Chawdhry, a top Health Ministry official. It also will contact people who have arrived from Mexico and other affected countries in the past 10 days to check for the symptoms, he said.
Some officials cautioned that the checks might not be enough.
The virus could move between people before any symptoms show up, said John Simon, a scientific adviser to Hong Kong's Center for Health Protection.
China, Russia and Ukraine banned imports of pork and pork products from Mexico and three U.S. states that have reported cases of swine flu, and other governments were increasing their screening of pork imports. Azerbaijan banned all livestock products from all of North America.
Indonesia, which was hit hardest by bird flu, said it was banning all pork imports. Lebanon said all pork products, except for some canned varieties, were banned.
The CDC says people cannot get the flu by eating pork or pork products.
Germany's leading vacation tour operators were skipping stops in Mexico City as a precaution. The Hannover-based TUI said trips through May 4 to Mexico City were being suspended, including those operated by TUI itself and also through companies 1-2 Fly, Airtours, Berge & Meer, Grebeco and L'tur.
TUI said other holiday trips to Mexico would continue to operate but would not make stops in Mexico City "for the next few weeks."
Japan's largest tour agency, JTB Corp., suspended tours to Mexico at least through June 30.
Russian travel agencies said 30 percent of those planning to travel to Mexico in early May had already canceled.
At Madrid's Barajas International Airport, passengers arriving from Mexico were asked to declare where they had been and whether they had felt any cold symptoms. They were told to leave a contact address and phone number.
"Where we were, there was no real alarm but we followed what was happening on the news and we're a little bit worried," said Spaniard Filomeno Ruiz, back from vacation in Cancun.
Passengers were also urged to contact health authorities if they notice any symptoms in the 10 days following arrival.
In the airport's baggage claim area, ground crews and police wore surgical face masks. Some travelers took precautions even though they had not been in Mexico.
"Nobody has recommended it, but I've put the mask on out of precaution," said Roger Holmes of Britain, who was traveling to Tunisia from Madrid. "I'm not afraid, but it costs nothing to be careful."
For facts about influenza, and more information about swine flu, please visit the Health Department and CDC websites. Some specific resources:
From New York City Health Department
Facts about flu
From Centers for Disease Control and Prevention
General information about swine flu
Swine Flu Case Definitions
Swine Flu Infection Control and Patient Care
Preventing the Flu
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