Train riders in Mexico City wear masks after outbreak of swine flu, 24 Apr 2009

Mexico's health minister says the swine flu outbreak may now be responsible for 81 deaths.

Jose Angel Cordova issued the revised death toll late Saturday as the Mexican government announced it was shutting down schools in the capital and surrounding areas until May 6.

With reports of more confirmed and possible cases from Mexico and the United States, the World Health Organization declared the virus "a public health emergency of international concern" with "pandemic potential."

The U.S. Centers for Disease Control, Interim Deputy Director Dr. Anne Schuchat, warned that officials "cannot contain the spread of this virus."

New Zealand's health minister Tony Ryall said Sunday 10 students who recently visited Mexico are "likely" to have contracted swine flu.

Officials in Mexico suspect more than 1,300 Mexicans have been sickened by the virus.  Sunday Mass has been suspended in many Roman Catholic churches throughout the country. 

Mexico City's mayor has canceled all public events for 10 days, and the country's health department has been given the power to isolate patients and inspect travelers.

In the United States, health officials confirmed three new non-fatal cases of the virus, one in California and two in Kansas, bringing the total to 11. 

Health officials in the southern U.S. state of Texas say they have temporarily closed a school outside the city of San Antonio after identifying possible cases of swine flu.

New York City's health commissioner said Saturday tests on eight of nine samples taken from sick students at one school came up "probable" for swine flu. But he said all of the possible cases were mild and that many of the children are feeling better.

The WHO is recommending that all countries intensify efforts to track its potential spread but there are growing concerns about the impact swine flu may have on air travel.

British Airways said Saturday a crew member was taken to a London hospital with flu-like symptoms after a flight from Mexico City. And Mexicana Airlines is giving international travelers the option of rescheduling flights to Mexico City at no extra cost.

Health officials say the unusual flu strain contains genetic material from pigs, birds and humans but authorities say none of the U.S. patients had any contact with pigs.

U.S. health officials say swine flu symptoms resemble the regular human seasonal influenza and include fever, lethargy, lack of appetite and coughing.