Indonesia, the world's most populous Muslim nation, has vowed to do more to crack down on terrorism following this month's suicide bombings on Bali island - the fourth major attack in the country since 2002.

"Almost all the people behind the last four terror attacks came from a hardline group that studied at certain Islamic boarding schools," Yusuf Kalla told reporters late on Wednesday.

"Out of the 17,000 boarding schools (in Indonesia), there are one or two, I repeat one or two, that are extreme and not in line with the teachings of Islam. We will have to investigate and control (them)."

Moves to monitor Islamic establishments are sensitive in Indonesia.

Suspected schools

Former dictator Suharto, who feared political Islam as a threat to his rule, ordered his security forces to keep a close eye on mosques and schools. Thousands of Muslim activists were imprisoned without trial until he was ousted in 1998 amid nationwide riots.

Kalla did not specify which boarding schools he suspected were spreading hardline teachings.

But two such schools - the Al-Mukmin and Al-Islam schools on the island of Java - have come under particular scrutiny in recent years after several bombers were found to have attended them.

Those bombers included activists convicted in the 12 October 2002 Bali bombings that killed 202 people, and the 2003 bombing of the JW Marriott hotel in Jakarta that killed 12.

Al-Mukmin was headed by cleric Abu Bakar Bashir, who some officials have called Jemaah Islamiyah's spiritual leader despite his recent acquittal on charges of running the organisation.