CILACAP (Indonesia) - THE execution of the three Bali bombers appeared imminent on Wednesday as wooden posts to secure them for the firing squads were erected on their prison island, an official source said.

Senior Indonesian Islamic clerics also said they been asked to visit the Islamist militants in their isolation cells on death row, to provide final spiritual counselling.

The official source said three wooden posts had been erected five metres apart, six kilometres south of the high-security prison on Nusakambangan island, southern Java, where the bombers are being held.

Officials have said only that the bombers - Imam Samudra, 38, Amrozi, 47, and his brother Mukhlas, 48 - will be shot in 'early Nov.'

The bombings of packed nightspots on the resort island of Bali in 2002 killed more than 200 people, mostly foreign tourists including 88 Australians, as well as 38 Indonesians.

Executions in Indonesia are carried out by firing squad in the dead of night at undisclosed locations. The prisoners are usually informed at least three days before.

Cleric Sahlan Natsir, a member of the local Islamic clerical body near the prison, said he was preparing to go to the jail to counsel the bombers before they die.

'We have received a letter from the Cilacap prosecutor's office to send nine clerics to the prison to provide counsel for the trio,' he said.

The letter did not specify when the clerics would be asked to visit the condemned men, who are said to be calmly awaiting death after being placed in isolation on Friday.

They are reportedly impatient to become 'martyrs' for the Islamic utopia they dream of creating across Southeast Asia.

Security has been tightened across the mainly Muslim archipelago amid fears of revenge attacks from Islamist extremists and the Jemaah Islamiyah regional militant network that allegedly instigated the Bali carnage.

Additional security has been given to President Susilo Bambang Yudhoyono after a threat to his life in the form of a letter purportedly penned by the bombers and posted on an Islamist website.

The letter dated Aug urges Islamist militants to 'war against and kill' Mr Yudhoyono, who is universally known by the initials SBY, and other senior officials in retaliation for the executions.

The US and Australian embassies were the target of anonymous bomb threats sent to police by text message on Tuesday, and on Wednesday another threat was reportedly received against the Australian mission.

Searches of the heavily guarded embassy compounds failed to find any bombs.

Islamic extremists descended for a second day on the village of two of the bombers, Amrozi and Mukhlas, shouting 'Allahu Akbar' (God is great) and threatening violence in response to the executions.

Around 60 white-clad radicals from the district around Tenggulun village in East Java arrived by motorcycle in the early afternoon and unfurled banners hailing the bombers as 'holy warriors.'

'Destroy the enemies of Islam, destroy SBY,' yelled radical cleric Muhammad Muchsin, referring to Mr Yudhoyono.

Australia has warned citizens against travel to Indonesia, and the US - which lost seven nationals in the attack - has urged Americans in the country to 'maintain a low profile.'

Anti-death penalty campaigners have questioned the legality of the executions, saying the bombers were convicted under a 2003 anti-terror law that was applied retroactively.

The Bali attacks were the bloodiest in a sustained period of Al-Qaeda-inspired jihadist violence in the world's most populous Muslim country.

Bombings at the JW Marriott hotel in Jakarta in 2003, the Australian embassy in 2004 and Bali again in 2005, among others, killed scores of people.