We know virtually nothing about the training that these people will have, what they're allowed to do on board and what sort of weapons they'll have. It is very worrying.

Mark Burgess, chief executive of Australian Police Federation

The Attorney General, Daryl Williams says targets may include American and British interests in Australia.

Mr Williams says the 22 Australian Air Security Officers, or air marshals, are ready to join the country's expanded security network.

"When and where they will be deployed is still the subject of discussions with the airlines," said Mr Williams.

Security cost

"They will be randomly placed and you won't be able to tell by looking at a person that yes, this is a security officer. They will look like an ordinary member of the public," he said.

The scheme to protect Australia's air passengers will cost up to $30m (US$15m). It is unclear who will pay for it.

Both the Federal government and the airlines are insisting it is the other's responsibility. If the airlines are forced to foot the bill, the costs will be passed onto passengers, with every journey costing $1 extra.

A passenger accused of carrying explosives in his shoes has again highlighted security

The Australian newspaper reports that civil liberties groups are questioning the legal basis of the marshals' power to carry weapons and arrest people on planes.

The secretary of the Australian Council for Civil Liberties, Cameron Murphy, said it was unclear what right the marshals had to act against suspects - arrest them or fire weapons.

The flying guards are selected from the ranks of the Government Protective Service. It is the government's own security force and looks after dignitaries and provides security at embassies and airports.

The use of its personnel as sky marshals has caused concern among police officers. Mark Burgess, the chief executive of the Police Federation, a union representing the majority of officers in Australia, says his members are more suitable for the job.

More planned

"We know virtually nothing about the training that these people will have, what they're allowed to do on board and what sort of weapons they'll have. It is very worrying."

Eventually the Australian Government plans to have more than 100 marshals on both domestic and international flights.

In a further move to reinforce in-flight security, Qantas has been given the go-ahead to strengthen the cockpit doors on its fleet.

In America, air marshals have been deployed since the 1970s. The US programme provides a covert, armed security force capable of carrying out anti-hijacking operations.

For many years, El Al, the Israeli airline, was the only carrier to have armed guards on all its flights as a matter of routine.