Sydney - Five Australians recently returned from either North America or Mexico and who developed flu-like symptoms are being tested for swine flu, authorities said Monday. New South Wales chief health officer Kerry Chant said the symptoms in the three adults and two children were mild.

"There have been some tests performed, but no cases meet the probable case definition, and there have been no confirmed cases," she said.

The captains of all passenger planes arriving in Australia from the Americas are now required to report on the health of their passengers. Any displaying flu-like symptoms will be seen by a quarantine officer, who will decide whether a medical examination is required.

A national hotline has been set up to field calls from people concerned about swine flu.

Virologist Greg Tannock from Melbourne's RMIT University warned that, with an effective vaccine against swine flu a year away, the outbreak could spread around the world.

"This is something that will play itself out over the next six months, especially in the southern hemisphere with the flu season coming along," he told national broadcaster ABC.

Tannock said that swine flu was more likely to become a pandemic than severe acute respiratory syndrome (SARS) or bird flu because it seemed more able to be transmitted between humans. (dpa)