HONG KONG (AFP) - Health authorities across Asia were scrambling Saturday to limit the spread of swine flu after reporting two confirmed cases in one of the world's most densely populated regions.

South Korea reported Saturday that a 51-year-old woman who recently visited Mexico tested positive for swine flu, while Hong Kong's first confirmed case was a 25-year-old Mexican who arrived in the city from Mexico via Shanghai.

The Hong Kong hotel where he had briefly stayed, along with 300 guests and staff, was cordoned off by police and put under a seven-day quarantine while other countries including India and Japan reported suspected cases.

Hong Kong's confirmed case, the first in Asia, sparked a regional health alert, with China immediately ordering health authorities to find and isolate the man's fellow passengers.

China "asked health authorities in Beijing, Shanghai and Guangdong to immediately put passengers... under quarantine and a seven-day medical observation," its health ministry said on its website.

Fears for a rapid spread of flu were heightened with China on Saturday entering the second day of an annual holiday that sees tens of millions of people on the move.

Authorities in Guangdong, which borders Hong Kong, said they were still looking for 11 people who had been on a flight with the Mexican after tracking down 30 other passengers, the official Xinhua news agency said.

A similar response was launched in Taiwan, where the government was trying to track down seven people who had travelled on the same flight as the Mexican.

The seven were among 25 people who had travelled from Shanghai to Taiwan via Hong Kong, Health Minister Yeh Chin-chuan told reporters.

Yeh said as of now the 18 people already identified "have shown no symptoms" of swine flu.

However, "sooner or later, we may have first confirmed cases since there is no way to restrict travel," Yeh said. "What is really important is to stop any community infection outbreak."

In Japan, also in the midst of a holiday period, the foreign ministry said a four-month-old US baby girl was being tested for swine flu, in the country's latest suspected case.

The baby had tested positive for the type-A virus after she arrived at Tokyo's US Yokota Air Base on a military flight from the United States with her family Friday, the ministry said.

Samples from the baby were sent to the National Institute of Infectious Diseases in Tokyo, and "they are now under examination to judge if they are the new flu virus," said foreign ministry official Toshiyuki Maeda.

In India, health authorities isolated two men at a hospital in New Delhi after they arrived on separate flights from abroad.

"Both of them are under observation in an isolation ward. We have done all the tests and samples have been sent to the National Institute of Communicable Diseases," said N.K. Chaturvedi, its medical superintendent.

One of India's suspected cases arrived on a flight from London overnight while the other travelled from Texas on April 19.

If confirmed they will be India's first cases of A(H1N1), the virus linked to swine flu, in a country of mover than 1.1 billion people.

Meanwhile, in New Zealand the number of suspected cases there had dropped, the health ministry said, adding that there was no evidence of swine flu spreading in the country.

New Zealand had reported four confirmed cases after a school party returned from Mexico earlier this week, on a flight from Los Angeles that landed in Auckland last Saturday.

However, Radio New Zealand quoted the ministry's national coordinator for pandemic planning, Steve Brazier, as saying "no swine flu is circulating in the community at present, that we've seen."