HONG KONG - A 27-year-old woman admitted to a Hong Kong hospital on suspicion of having contracted swine flu in the United States was declared free of the virus, a health official said Monday.

The woman, who had just returned from San Francisco, underwent tests after showing flu-like symptoms, Thomas Tsang, controller of the Centre for Health Protection, told reporters.

However, tests found that she had contracted human flu, not swine flu, according to government statement issued later Monday.

She is the third person to be cleared of swine flu since Hong Kong was put on the alert for the disease, following Sunday's outbreak in Mexico that may have killed more than 100 people.

Tsang said that a 77-year-old woman who had just returned from a trip to Mexico and the United States had also been examined by doctors, along with her granddaughter, who had not travelled outside Hong Kong.

Both showed some flu symptoms but were released after being given the all-clear, Tsang said.

The suspected cases come as Hong Kong, at the forefront of the SARS epidemic in 2003 and already on alert for bird flu, has announced a series of tough measures to combat any threat of swine flu.

The government changed the law Monday to require doctors to report suspected cases of human swine flu to local authorities, after making it a "notifiable disease."

As a result, authorities now have the authority to quarantine anyone who has come into contact with a suspected patient.

Gabriel Leung, a senior official at the health department, said he thought Hong Kong was one of the first territories in the world to take the measure for swine flu.

Authorities said earlier they would detain anyone showing symptoms of the virus after arriving from an infected area.

Health officials have also advised against all non-essential travel to worst-hit Mexico. Cases are also being investigated in Europe, the United States, the Middle East and New Zealand.

Tsang said Sunday that protection measures, including the use of temperature screening machines at airports and other entry points, had been stepped up.

"If any passenger fails the temperature test... he will be interrupted and we will obtain the history whether in the past seven days he has been to any of these places affected by swine flu," he told reporters.

"If that history is positive, we will take that patient to the hospital and let him stay there and have a test -- and until the test result is negative, we won't allow him to get out of the hospital."

Messages were being broadcast on inbound flights, especially those arriving from affected areas such as Mexico and the United States, advising passengers to report symptoms such as sudden fever.

Hong Kong Chief Executive Donald Tsang said the government would closely monitor the situation.

"We are going to regularly report to Hong Kong people how this thing is evolving, particularly various preventive measures we are taking," he told reporters.

Lo Wing-lok, a doctor and bird flu expert, said Hong Kong was in a good position to cope with swine flu, following its SARS ordeal.

"Other countries in the region could certainly learn from Hong Kong's painful and bitter experience in dealing with the SARS and bird flu outbreaks," he told AFP.