U.S. Embassy spokeswoman Susan Stevenson told ABC News that initially a total of four Americans were put unde quarantine.
China also banned pork imports, despite continued assurances from health officials that the virus cannot be contracted by eating pork.
Mexican President Felipe Calderon, angry at reports that the Chinese had rounded up Mexicans in China even if they were not sick, and at several other countries that had banned flights to Mexico, said late Monday in a televised address that the world should "stop taking actions that only hurt Mexico and don't contribute to avoid the transmission of the disease."
A government charted plane from Mexico City arrived in China Tuesday to pick up Mexicans who were held in quarantine and wished to leave.
The United States has 380 confirmed cases in 36 states but still only one death, leading some experts to say the bug may be waning here.
In Geneva, Switzerland, today more than 150 experts will meet to compare notes on the H1N1 flu virus, including its severity and incubation period, WHO said.
Also Tuesday the United Nations will start sending enough of the anti-viral drug Tamilflu to treat 2.4 million people in 72 countries.
Some experts worry that as winter approaches in the Southern Hemisphere seasonal flu outbreaks could trigger a resurgence in the swine flu. New Zealand is the only country in the Southern Hemisphere that has a confirmed case of swine flu.
As for the CDC's analysis of the H1N1 virus, Besser said it still has not found traits condusive to pandemic caliber outbreaaks. "What we've found is that we're not seeing the factors that were associated with the 1918 pandemic, we're not seeing the factors that are, were associated with other H1N1 viruses, and that's encouraging."
Even though officials are cautiously optimistic, whether the virus will re-emerge when the typical flu season starts in the fall is still unclear.
"Every virus is new," Besser cautioned. "And what it will do is different. And so you're hitting a critical point: What will happen this spring and summer?"
He reiterated that the virus is spreading "quite easily," and that he expects to see reports of more confirmed cases in the United States.
In New York City, St. Francis Preparatory School -- last week considered the epicenter of the country's swine flu infections -- reopened Monday for the first time since the virus was first identified.
As students returned to the school, many packing bottles of hand sanitizers in their backpacks, medical experts concluded that the swine flu infection may not be any more dangerous than a normal outbreak of flu.
But the country's medical authorities aren't ready to ease restrictions, and more than 333,000 students were out of class Monday as schools across the country took no chances and shut down. Two more New York schools closed their doors Monday, including one in Syracuse and another on Long Island.