South Korea Saturday reported its first case of swine flu after Hong Kong said a man who flew from Mexico had the disease, in Asia's first confirmed cases as the virus continues its global spread.
But health authorities said the world appeared better prepared to fight an epidemic than a few years ago, and vowed that a vaccine was only months away.
Confirmation by Hong Kong authorities Friday that a traveller who arrived from Mexico, via Shanghai, had tested positive for A(H1N1) flu virus saw an entire hotel quarantined.
And in South Korea, authorities Saturday confirmed the country's first case, involving a a 51-year-old woman who returned from Mexico Sunday.
The Hong Kong case sent shivers through the territory as thousands donned masks in an echo of the 2003 SARS crisis.
China responded quickly, saying it was to put all passengers on the flight from Mexico to Shanghai under quarantine and that flights from Mexico to the city were being suspended.
Denmark and France joined the list of countries reporting their first cases.
But in a sign that authorities may be containing the spread of the disease, Mexico, which has been at the epicentre of the outbreak, said the new, multi-strain virus is not so aggressive as had first been feared.
Mexico's government late Friday raised its confirmed toll of flu cases to 16 dead and 381 infected.
"Fortunately, the virus is not so aggressive, it's not a case of avian flu, which had a mortality rate of nearly 70 percent," Health Minister Jose Angel Cordova earlier told reporters.
Hong Kong Chief Executive Donald Tsang was taking no chances, however, in this densely populated, sub-tropical territory, saying he would "raise the alert level from serious to emergency."
China "has asked health authorities in Beijing, Shanghai and Guangdong to immediately put passengers on flight AM098... under quarantine and a seven-day medical observation," the health ministry said on its website.
And Xinhua news agency reported Beijing was shutting down flights between Mexico and Shanghai.
Xinhua reported that China had notified the Mexican government and airlines about the decision, adding Beijing may send a charter plane to Mexico to collect "Chinese passengers who had planned to fly to Shanghai from the city of Mexico on May 3."
Meanwhile, the Korea Centers for Disease Control and Prevention (KCDCP) said in a statement: "Test results have confirmed the first probable patient, whose case was reported on Monday, was infected with Influenza A (H1N1)."
The woman, reportedly a nun, who returned from Mexico Sunday, has been responding well to treatment and is expected to be discharged from hospital Monday, health officials said.
The World Health Organisation (WHO) has warned that a pandemic is now imminent, raising its alert level to five out of six, but a senior official at the UN agency said a Vaccine was in the pipeline.
"We have no doubt that making a successful vaccine is possible in a relatively short period of time," Marie-Paule Kieny, WHO director of the Initiative for Vaccine Research said, adding it may take four to six months.
With 143 infections confirmed in the United States across 20 states, US officials also said the outbreak did not appear to be anywhere near as dangerous as the 1918 flu epidemic, killing an estimated 50 million people around the globe.
"We do not see the markers for virulence that were seen in the 1918 virus," Nancy Cox, the chief of the Centers for Disease Control and Prevention (CDC) influenza division, told reporters.
The head of the Pan American Health Organisation (PAHO) added her voice to global reassurances, saying Latin America was better prepared to handle today's outbreak than five years ago, when avian flu struck.
Spain, meanwhile, reported that its number of suspected swine flu cases appeared to have stabilised as the figures dropped to 108 from 116. There are so far 13 confirmed cases in the country.
Among the latest cases revealed was that of a nurse in Germany who had treated a patient with the disease, but had not been to Mexico.
Scottish authorities also confirmed the first case of swine flu in Britain involving someone who had not recently travelled to Mexico.
And in France Health Minister Roselyne Bachelot said the country's first two cases of swine flu had been confirmed.
Most cases outside Mexico have involved only mild symptoms of the illness that can be easily treated with existing flu medicines, and some experts have suggested the virus may have weakened as it was carried outside the country.