Schools would also close in San Luis Potosi, the third most affected area, in central Mexico, Cordova said.

Disease control

Felipe Calderon, the Mexican president, issued an emergency decree on Saturday giving the government special powers to run tests on sick people and order them to be isolated in an attempt to control the outbreak.

The decree, published in Mexico's official journal, gives the government power to isolate sick people, enter homes or workplaces and regulate air, sea and land transportation to try to stop further infection.

Franc Contreras, reporting for Al Jazeera from Mexico City, said the authorities had advised people not to go outside unless necessary and that many residents had bought surgical masks in an attempt to avoid the disease.

Two of Mexico's main football games, one of them in the capital's giant Aztec Stadium, will be played without spectators on Sunday to avoid large crowds.

Armando Ahued, Mexico City's health minister, has said a vaccination campaign was being launched against the illness.

Dr Myles Druckman, a US pathologist who specialises in planning for disease outbreaks and pandemics, told Al Jazeera: "The whole concept of an influenza pandemic and how it is spread is that it is spread from person to person.

"Really, the strategy that most public health authorities have is to try to reduce large social gatherings - social distancing, they call it. Some of these processes can be draconian but it really needs to be evaluated based on how the virus is functioning on the ground."

WHO warning

Earlier on Saturday, the World Health Organisation (WHO) declared the outbreak of the A/H1N1 virus, which is a mix of human, pig and bird viruses, a "public health emergency".

Chan has urged countries to increase
vigilance following the outbreak [AFP]

Margaret Chan, the WHO chief, described the outbreak as "serious" and called on countries to "increase vigilance" following the discovery of related strains on both sides of the Mexico-US border.

Chan said how the situation would evolve was "unpredictable" but that it has "pandemic potential".

An emergency committee of the WHO met in Geneva on Saturday to oversee the agency's handling of the outbreak.

Tests by the WHO on Friday showed the virus from 12 of the Mexican patients was the same genetically as a new strain of  H1N1.

US cases

At least two cases of the human swine influenza have been confirmed in Kansas and one more in California, bringing the number of people infected by the virus in the US to 11.

At least eight more cases are suspected in students at a New York City high school, but health officials said on Saturday they don't know whether they have the same virus that has killed scores of people in Mexico.

Kansas health officials said on Saturday they had confirmed swine flu in a married couple living in the central part of the state after the husband visited Mexico.

The couple were not hospitalised, and the state described their illnesses as mild.

At least nine swine flu cases also have been reported in California and Texas. The new California case, the seventh there, was a 35-year-old woman who was hospitalised but who has recovered.

New York health officials said that more than 100 students at the private St. Francis Preparatory School, in Queens, had come down with a fever, sore throat and other aches and pains in the past few days. Some of their relatives also have been ill.

New York City Health Commissioner Thomas Frieden said nose and throat swabs had confirmed that eight students had influenza type A, indicating probable cases of swine flu, but the exact subtypes were still unknown.

US alert

US officials said the White House was closely following the outbreak and that Barack Obama, the president, has been informed.

The European Centre for Disease Prevention and Control said the A/H1N1 influenza identified in at least two of the recent cases by US counterparts could certainly develop into a pandemic-type virus.

Prior to the current outbreak, the United States had seen 12 cases of the human form of swine influenza since 2005.

In recent years the global focus for a possible pandemic has shifted to the H5N1 bird flu virus, which has spread from poultry to humans and killed 257 of the 421 people infected by the virus since 2003.