Australia's Foreign Minister Alexander Downer has warned that "a small number" of Australians have joined al-Qaeda.
He was speaking about a videotaped message broadcast this week in which a man with an Australian accent threatened attacks against the West.
Australian papers have identified the man, who was dressed in combat gear, as former soldier Matthew Stewart.
"We have reason to believe he is one of a number of Australians who have turned to al-Qaeda," Mr Downer said.
The video, first broadcast by Dubai-based Al-Arabiya television, shows a man wearing combat gear and a balaclava, and carrying a rifle.
"As you kill us, you'll be end. As you plant us, you will be planted," the man warned, without saying who he represented.
He also criticised US President George W Bush and British Prime Minister Tony Blair.
Following the broadcast, Australian authorities visited Mr Stewart's family, who have not seen him in four years.
A statement issued by the family denied the man in the video was Matthew Stewart, who left the Australian army in 2001 after serving in East Timor.
Iraq coalition
Mr Downer said Mr Stewart was believed to have links with terrorism.
"He is one of a small number of Australians we have had concerns about in this respect," he said, refusing to specify the exact total.
"Any Australian who thinks that joining al-Qaeda is a way for the future is an Australian who is taking up arms against the Australian people - and will find themselves, if captured, in enormous difficulty," Mr Downer said.
The video's release comes as Australian Prime Minister John Howard is pushing to tighten security laws.
Australian troops were part of the coalition that invaded Iraq, and have also operated in Afghanistan.
