Mexico canceled classes for millions of children in its sprawling capital city and surrounding area on Friday after influenza killed more than 20 people in recent weeks.
The World Health Organization said it was concerned over what it called 800 "influenza-like" cases in Mexico, and also about a confirmed outbreak of swine flu in the United States.
US health authorities were unsure whether the outbreak of a new kind of swine flu in California and Texas, which have borders with Mexico, was related to the unusually late and severe flu season in that country.
Mexican authorities say the flu outbreak has killed around 20 people, but WHO said about 60 people have died in the country.
The Geneva-based UN agency said it was in daily contact with US, Canadian and Mexican authorities and had activated its Strategic Health Operations Center (SHOC), its command and control center for acute public health events.
There have been some 800 suspected cases with flu-like illness, with 57 deaths in the Mexico City area, and a further 24 suspected cases and three deaths in San Luis Potosi in central Mexico, WHO spokeswoman Fadela Chaib said.
Mexican Health Minister Jose Angel Cordova said schools and universities in Mexico City and the surrounding area would be temporarily closed on Friday and advised people with flu symptoms to stay home from work.
"We're dealing with a new flu virus that constitutes a respiratory epidemic that so far is controllable," Cordova said.
Some 20 million people live in Mexico City and the surrounding metropolitan area.
Canada's government advised doctors to be on the alert for reports of illness from people who recently traveled to Mexico, although it did not advise against visiting the country, a popular destination for beach vacations.
Flu season usually ends earlier
Mexico's flu season normally ends in February or March, but it has lasted longer this year, the government said.
"We recommend avoiding places or events with a lot of people unless strictly necessary," Cordova said in an unusual late-night live statement to media.
About 79 people, possibly ill with the flu, are being treated in Mexico and that number has not increased in recent days, the health ministry said.
Worldwide, seasonal flu kills between 250,000 and 500,000 people in an average year.
In the United States, seven people have been diagnosed with a new kind of swine flu in California and Texas, the US Centers for Disease Control and Prevention reported.
All seven people there have recovered but the virus itself is a never-before-seen mixture of viruses typical among pigs, birds and humans, the CDC said.
"We are likely to find more cases," the CDC's Dr Anne Schuchat said. "We don't think this is time for major concern around the country.
Canadian officials have been particularly sensitive to the international spread of respiratory illnesses since Toronto was hit by the SARS epidemic in 2003, which was blamed partly on a slow response to early disease reports.