SYDNEY, April 27, 2009 (AFP) - The swine flu outbreak is more worrying than bird flu because it is spread much more readily between humans, an Australian infectious diseases expert said Monday.

Australian National University epidemiology specialist professor Paul Kelly said swine flu had a lower mortality rate than bird flu but warned this was a mixed blessing because it would help the virus spread more quickly.

He said bird flu had remained relatively contained because human-to-human transmission was difficult, while swine flu was highly infectious.

"(Bird flu) has been limited - to a limited extent that has happened in Indonesia and other places, but it's never been on the sort of scale as this," Kelly told ABC radio.

"This is actually really more worrying."

He said swine flu appeared to be a form of the virus that epidemiologists had feared for years - a combination of strains from various animals that was easily transmitted between humans.

"In terms of an epidemic, for the virus to be able to spread it's actually better for the virus for humans to remain alive because that can spread it more quickly and to a greater extent geographically," he said.

Professor John Mackenzie, a biosecurity expert from Perth's Curtin University, said the latest flu threat appeared to be a combination of at least two types of swine virus and an avian virus gene.

He said the next few days would be crucial in determining whether the world was facing a pandemic.

"I guess we're at that 'grey' stage where we don't know if it is going to be a pandemic strain or not," he said.

"We're certainly concerned but at the same time Australia is in a better position than most other countries to be able to withstand or cope with a pandemic."