Australia's Department of Foreign Affairs is encouraging people travelling to Athens for the August 13-29 Games to be aware of the increased terror threat globally and to register their details with the department.

"Australians in Greece are advised to exercise caution and keep themselves informed of developments that might affect their safety," the department said on its website (www.dfat.gov.au) on Wednesday.

"Australians should be aware of a recent series of firebomb attacks on domestic political and commercial premises in Greece, mainly in Athens and Thessaloniki. More recently there have been incidents involving improvised explosive devices."

Australian radio reported that Greece's Public Order Minister Yiorgos Voulgarakis had described the travel warning as "exaggerated" and "empty of substance".

Three small bombs exploded outside a central Athens police station a week ago. Greek authorities have blamed the blasts on local anarchists and described them as "meaningless".

But some Olympics heavyweights, including Australia and Japan, have since begun reviewing security plans for the Games.

Greece has set up a security group comprising the United States, Britain, Israel, Spain, France, Germany and Australia to protect the Games, the first to be staged since the September 11, 2001, attacks on the United States.

Greece expects to spend about one billion euros ($1.18 billion) on security for the August games, almost five times the amount spent for the 2000 Olympics in Sydney.

Its overall plan encompasses more than 45,000 armed guards, NATO air and sea patrols, thousands of closed-circuit cameras and restrictions around Olympic venues.

Australia has angered several of its Asian neighbours with a raft of travel warnings in the wake of the September 11 attacks and the 2002 nightclub bombings on the Indonesian island of Bali which killed 202 people, 88 of them Australians.