The UN’s World Health Organization has raised the alert over swine flu to level five – one short of a full-blown global epidemic, or pandemic.
A phase five alert means human-to-human transmission in at least two countries.
The move comes after a 23-month-old Mexican child died in Texas – the first death from swine flu outside Mexico, where the outbreak originated.
In Spain, officials confirmed the first case of swine flu in a person who had not travelled to Mexico.
Mexico’s President Felipe Calderon addressed the nation late on Wednesday, announcing the partial suspension of non-essential work and services from 1 May to 5 May.
The efforts of the government were concentrated on containing the virus, Mr Calderon said, urging people to stay at home with their families during the shutdown.
Mexico: 159 suspected deaths – seven confirmed
US: one death, at least 91 confirmed cases
Canada: 13 confirmed cases
UK: 5 confirmed cases
Spain: 10 confirmed cases
Germany, New Zealand: 3 confirmed cases each
Israel: 2 confirmed cases
Austria: 1 case
He said he was “proud” of the response of Mexicans to the crisis, and assured people Mexico was well-stocked with anti-viral medicines.
Announcing the latest alert level after an emergency WHO meeting in Geneva, Director General Margaret Chan urged all countries to activate their pandemic plans, including heightened surveillance and infection-control measures.
She said action should be undertaken with “increased urgency”.
She added: “It really is the whole of humanity that is under threat in a pandemic.”
But she also said the world was “better prepared for an influenza pandemic than at any time in history”.
Ms Chan said that for the first time, the pandemic could be tracked in real time.
This was necessary, she added, because the virus could mutate at any time into a more dangerous strain – or a milder one.
The Mexican boy who died in the US fell ill during a visit to relatives in southern Texas earlier this month.
He was transferred to a hospital in Houston, where he died on Monday night.
Speaking in Washington, President Barack Obama offered his condolences and said the federal government was doing the utmost to contain the virus.
He also urged local public-health bodies to be vigilant and said schools with confirmed cases “should consider closing”.
Officials put the number of suspected deaths from swine flu in Mexico at 159, although just seven deaths have been confirmed, with 26 infections positively tested.
Texas Governor Rick Perry said closing the US border with Mexico was an option, but added that taking that step now would be “a little premature”.
Giving a televised news conference on Wednesday evening, US President Barack Obama said health officials were not recommending closing the border.
“The key now is to just make sure we are maintaining great vigilance, that everybody responds appropriately when cases do come up,” Mr Obama said.
Since the virus emerged last week, it has also spread to Canada, Europe, Israel, and New Zealand.
Peru became the latest country to confirm it was treating a patient suffering from swine flu. An Argentine woman who had recently travelled to Mexico was Peru’s first case of the virus, the country’s health minister said late on Wednesday.
Several countries have restricted travel to Mexico and many tour operators have cancelled holidays.
France will ask the European Union on Thursday to suspend all flights going to Mexico because of the flu outbreak, Health Minister Roselyne Bachelot said.
The WHO, however, says measures like travel bans are unlikely to prove effective.
In Spain, the government said the first person to contract swine flu without having travelled to Mexico was the boyfriend of a young woman who had recently returned from there.
Spanish Health Minister Trinidad Jimenez said such cases were to be expected.
In total, the number of confirmed cases in Spain rose from two to 10 on Wednesday. None of the patients is seriously ill.
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
In Mexico, the search for the source of the outbreak continues, with the focus on the vicinity of a pig farm in the eastern part of the country.
The Mexican government is urging against jumping to conclusions and is suggesting the possibility remains that the virus originated outside the country.
Schools across Mexico have closed, public gatherings are restricted and archaeological sites have been placed off-limits.
Mexico City’s chamber of commerce estimated restrictions in the city were costing businesses there at least 777 million pesos ($57m, Ã‚Â£39m) per day.
WHO official Keiji Fukuda said other countries also needed to consider “social distancing” measures such as closing schools and delaying public meetings.
Meanwhile, Ghana has become the latest country to ban pork imports as a precaution against swine flu, though no cases have been found in the West African country.
Ms Chan, the WHO director, stressed on Wednesday that there was no danger from eating properly-cooked pork.
She advised hygiene measures such as hand-washing to prevent infection and said it was important “to maintain a level of calm”.