He returned on Emirates flight EK405 and was seated in row 18. The Ministry of Health (MOH) is now tracking those who had come into contact with him.

Melbourne was recently added to MOH's list of affected areas, which include the United States, Canada, Mexico and Chile, as well as Kobe and Osaka in Japan. Singaporeans are advised to avoid non-essential trips to these areas.

For the 10th and 11th patients, the ministry has tracked down all those who had been in close contact with them.

They had arrived here on Monday on Singapore Airlines SQ25 from New York via Frankfurt, the same flight as the country's seventh case - a 30-year-old Australian man who fell ill before flying.

The 10th patient, a 33-year-old Singaporean woman who had been in the US between May 23 and 30, had developed symptoms on Monday and was admitted to the Communicable Disease Centre 2 before midnight on Tuesday.

The ministry has identified two people who had been sitting near her seat in row 19 and will quarantine them, as well as two others living with her.

The 11th patient, an 18-year-old female US visitor who came to visit a relative, had developed symptoms more than 24 hours after getting off the plane.

As patients are believed to be infectious within 24 hours before symptoms appear, she is unlikely to have spread the virus to her fellow passengers, MOH said. MOH did not contact any passengers sitting near her seat in row 57. Four people living in the same household as her have been placed under quarantine.

So far, more than 120 people have been ordered to stay indoors for being in close contact with infected patients.