Indonesian anti-terror police are questioning four foreign nationals in a sign of tightening security in the country with the world's largest Muslim population, officials confirmed on Tuesday.
Indonesia grappled with a string of militant attacks during the 2000s, including a bombing at a Bali nightclub that killed 202, with 88 Australians among them. Since then, it has been largely successful in cracking down on homegrown militant cells.
Although police commonly arrest Indonesians suspected of extremist activities, it is rare for them to detain foreigners, and may signal they suspect a new tactic by militants.
"At the moment we are still investigating where the foreigners are from and what their objective was," Marciano Norman, head of Indonesia's intelligence agency, told reporters.
He gave no further details about the four people arrested, but appealed for the public to be vigilant for suspicious foreigners.
Police on Saturday arrested the four and seized fake passports from them on the eastern island of Sulawesi, which is the headquarters of a homegrown, but weak, radical Islamic group.
The Indonesian government has raised concern over a possible spillover from the hardline Islamic State in the Middle East, after revelations that Indonesian citizens had traveled to Syria and Iraq to join fighters there. Islamic State teachings and endorsement were banned last month.
But Indonesia has stopped short of backing U.S.-led military action against Islamic State.
"President Susilo Bambang Yudhoyono has emphasized the approach should be comprehensive, not just about force and violence, but also political resolution," said Foreign Minister Marty Natalegawa.