MEXICO CITY — With swine flu still spreading, the U.N. health agency is warning countries to limit their use of antiviral drugs to only high-risk patients to ensure adequate supplies in case the virus should mutate and become more dangerous.

The global outbreak appears mild, but skittishness is evident. Not long after Switzerland lifted its advisory against travel to Mexico and the United States on Tuesday, the Japanese national women's soccer team canceled a tour to North America, where most swine flu cases have been reported.

In Mexico's Baja California state, on the U.S. border, 5,689 children were turned away from schools when classes resumed Monday because they had symptoms like runny noses, headaches or sore throats, the state education department reported Tuesday.

Three more nations — Cuba, Thailand and Finland — reported their first confirmed swine flu cases, all in people who had traveled to Mexico. A 24-year-old Hong Kong man returning from the United States was confirmed today as the Chinese territory's second case of swine flu, authorities said.

And China confirmed its second case of swine flu on the mainland, in a man who had recently returned from Canada.

There are now 33 countries reporting an estimated total of 5,916 confirmed swine flu cases, including 3,009 in 45 U.S. states, 2,282 in Mexico and 358 in Canada.

The death total is relatively low — 63, of which 58 were in Mexico, three in the U.S., one in Canada and one in Costa Rica.

But health experts worry about the chance the virus might become more lethal in the coming months, saying it is important not to overuse antiviral drugs since supplies are limited.

The World Health Organization said Tuesday antiviral drugs should be given to only patients most at risk.

Its comments appeared aimed at European countries, which have been using antiviral drugs such as Tamiflu and Relenza much more aggressively than the U.S. and Mexico, administering them whenever possible in an attempt to contain the virus before it spreads more widely.

Officials from EU and Latin American nations, including Mexico, were meeting in Prague today to discuss ways of combating the threat.

A WHO medical expert, Dr. Nikki Shindo, said the U.N. agency thinks antivirals should be targeted mainly at people already suffering from other diseases or complications — such as pregnancy — that can lower a body's defenses against flu.

The U.S. Centers for Disease Control and Prevention also said pregnant women in particular should take the drugs if they are diagnosed with swine flu — even though the effects on the fetus are not completely known.

Pregnant women are more likely to suffer pneumonia when they catch flu, and flu infections have raised the risk of premature birth in past flu epidemics. A pregnant Texas woman who had swine flu died last week, and at least 20 other pregnant women have swine flu, including some with severe complications.