The Australian government said it appears to be authentic, but local leaders have warned against panic in the southern city, saying the state’s security will never be compromised.
The 11-minute-long tape was aired on the American ABC television network after it was sent to the broadcaster’s office in Pakistan.
“Yesterday, London and Madrid. Tomorrow, Los Angeles and Melbourne,” said the masked speaker, believed to be American Islam convert Adam Gadahn.
“At this time, don’t count on us demonstrating restraint or compassion,” the speaker continued.
“We are Muslims. We love peace, but peace on our terms, peace as laid down by Islam not the so-called peace of occupiers and dictators.”
A police spokesman for the Australian state of Victoria, of which Melbourne is the capital, said Australian authorities are waiting to receive a copy of the video to verify its authenticity.
“We are working with our federal and international counterparts to obtain a copy of the tape,” Inspector Craig Walsh said.
“Once obtained, the tape will need to be authenticated and its contents analysed and considered prior to making any further statements.”
Federal Attorney-General Philip Ruddock said he is taking it seriously, however Australia’s terrorist alert would remain at medium.
“The threat is a generalised threat … it’s a generalised threat to Melbourne. We have no specific information that would cause us to change the alert at this time.”
Australian Prime Minister John Howard said the tape could be authentic.
“Clearly our country has been a terrorist target long before the 11th of September, 2001. Events such as this, whether the tape turns out to be genuine or not, only serves to remind us of the changed world in which we now live as a result of terrorist attacks,” he said, speaking from New York.
Mr Gadahn made a similar tape last year that US intelligence said was authentic, but Los Angeles’ police chief said he seemed to be “a mouthpiece, a spokesman, not an operative” of Osama bin Laden’s organisation.
US authorities in Los Angeles told ABC News that strong measures are already in place and that there is no need to impose additional security in the absence of any known credible threats against the city.
“We have a very robust counter-terrorism operation here in LA, similar to New York and Washington because we are always a prime target,” Los Angeles Police Chief William Bratton said.
The videotape statement, coming on the fourth anniversary of the September 11 attacks, underscored memorials in the cities of Washington and New York.
US President George W Bush led the nation in a minute’s silence on the lawn of the White House at the time the first hijacked aircraft struck New York’s World Trade Centre at 8:46 am (10:46 pm AEST).
Crowds gathered at Ground Zero, the site where 2,749 people were killed when the two towers of the trade centre collapsed in downtown New York.
A roll call of the victims was read out and sympathies were also extended to the country’s hurricane-ravaged Gulf Coast.