SYDNEY--Australians were warned Sunday against travel to Indonesia as execution loomed for three Islamist militants convicted of the 2002 Bali bombings that killed 202 people, mostly foreign holidaymakers.

An extremist backlash against the executions is feared and Foreign Minister Stephen Smith said if Australians insisted on travelling to Indonesia they should stay clear of places that could be targeted for attack.

"We are making sure that we advise Australians firstly to reconsider their need to travel to Indonesia," Smith told Nine Network television.

"We also indicate to them that if they do travel to Bali and Indonesia, to keep away from sites which have obviously been terrorist sites in the past."

The execution of the condemned men -- Amrozi, 47, his brother Mukhlas, 48, and Imam Samudra, 38 -- is believed to be imminent.

They have been placed in isolation and the order for their death by firing squad had been delivered, a source at the Nusakambangan island prison off southern Java has said.

Security forces have been placed on high alert across the mainly Muslim country as a precaution against an explosion of Islamist anger at the first executions to be carried out under Indonesia's anti-terror law.

Sensitive areas like foreign embassies, tourist spots, shopping malls and ports were under close guard. On the Hindu-majority resort island of Bali, 3,500 police were on the streets providing additional security, officials said.

Hand-written posters have appeared in the East Java city of Surabaya promising retaliation for the bombers' execution. "One word for Amrozi's killers: retribution," said one.

The 2002 bombings targeted nightspots packed with Western tourists, killing more than 160 foreigners including 88 Australians. The bombers said they were retaliation for the US-led wars in Afghanistan and Iraq.