The federal government and the AFP have refused to say whether former Army private Mathew Stewart is the masked man speaking in an apparent Australian accent shown in the video broadcast earlier this week.
Mr Stewart served with the Australian army in East Timor, and after being discharged left Australia for Afghanistan in August 2001 and has not been heard from since, according to his mother, Vicki Stewart.
In a media statement she said the family has been questioned by federal police about the man in the video, which was shown on Arabic-language al-Arabiya television.
But she denied the man in the video was her son, saying the family believed Mathew was dead.
Australian Attorney General Philip Ruddock on Friday warned that the reports identifying Mr Stewart as the militant are speculative and said Australian security agencies are still investigating the video.
“I can’t tell you whether (the media reports) are right or wrong. I can’t tell you whether it is an Australian or not,” Mr Ruddock said.
“Competent agencies are still undertaking a very thorough examination of the issue,” he said.
In the video, the masked militant denounced British Prime Minister Tony Blair and US President George W Bush while vowing new attacks on Western targets.
“The honourable sons of Islam will not just let you kill our families in
Palestine, Afghanistan, Kashmir and the Balkans, Indonesia, the Caucuses and elsewhere,” the masked militant said.
“It is time for us to be equals. As you kill us, you’ll be killed. As you bomb us, you will be bombed,” the militant said.
Foreign Minister Alexander Downer said Mr Stewart is one of a small number of Australians with suspected terrorist links.
“We have reason to believe he is one of a number of Australian who have turned to al-Qaeda,” said Mr Downer, quoted by AAP.
However he refused to say how many Australians are being monitored by ASIO over their militant leanings.