Support for groups like the Islamic State of Iraq and the Levant (ISIL), Sunni radicals currently rampaging through northern Iraq, is growing among Indonesian extremists with dozens believed to have joined the insurgency.
Analysts say the fighters will pose a new and serious threat to Indonesia when they return home with honed insurgency tactics and international militant connections, echoing the concerns of Western governments.
Britain and Australia have expressed fears that Syria and Iraq are breeding grounds for violent fanatics who travel there from the West to fight and pose a threat to national security on their return.
Indonesia, the world's biggest Muslim-majority nation, has long struggled with terrorism but a successful clampdown in recent years has seen the end of major deadly attacks -- ironically fuelling interest in Syria and Iraq.
"There's not much going on with jihad in Indonesia for militants anymore," said Taufik Andrie, a terrorism expert at the Institute for International Peacebuilding.
"There are just splinter groups with no resources or support, so many are inspired by what's going on in Iraq and Syria," he told AFP.
"When they return, they will be seen as high-profile jihadi. Young people will come to them for training, to form new groups, to plan attacks, to teach how to fight and make bombs."
Support for ISIL 'growing'
Indonesia's anti-terror unit has acknowledged that support for ISIL is growing, judging by rallies, social media and the sermons of radical preachers.
The militants have crossed the border from Syria into northern Iraq and taken over key cities in lightning gains, bringing the Iraqi army to its knees.
Indonesia estimates that 60 Indonesians have travelled to Syria and Iraq to fight but experts say the figure is closer to 100 and growing fast.
There are no laws prohibiting Indonesians from joining foreign militant groups and Islamic organisations have openly held fundraisers for ISIL.
"The government must pass legislation to criminalise citizens supporting and travelling overseas to join terrorist groups," Rohan Gunaratna, a terrorism expert at Singapore's S. Rajaratnam School of International Studies, told AFP.
Analysts point to neighbouring Malaysia which has been more active, arresting a dozen men in April who were trying to leave the country to fight in Syria.
It failed, however, to prevent a 26-year-old Malaysian from leaving the country for Iraq, where he carried out a suicide attack that killed 25 soldiers.
Indonesians know well the threat of returnees -- many of the country's most notorious terrorists trained in Afghanistan in the 1980s and 1990s and came back with vast networks, bomb-making skills and access to funding.
Some were in the al-Qaeda-linked Jemaah Islamiyah, which was behind the 2002 twin bombings that killed 202 people on the resort island of Bali as well as other blasts on hotels and churches.
The Bali bombings were a wake-up call that galvanised the government. An elite anti-terror police unit was established which has eliminated the masterminds of the attacks in bloody armed raids.
Syria reigniting jihadist interest
The country has had no significant bombings for around five years as the JI network crumbled, leaving only splinter groups and small cells with little capacity.
But the civil war in Syria has reignited interest in jihad as some Muslims believe it is the start of the Islamic equivalent of Armageddon.
"Some jihadists in Indonesia see ISIL as the embryo of an Islamic caliphate, which is their ultimate goal," said Solahudin, author of "The Roots of Terrorism in Indonesia" who goes by one name.
The fighters, almost entirely young men, are being wooed online. YouTube pulled a video from two channels Thursday of five Indonesian men in balaclavas claiming to be in Syria, calling for their compatriots to join the fight.
Radical Islamic websites, such al-Mustaqbal and VOA Islam, are publishing pro-ISIL news stories, describing its takeover of Iraqi cities as the "liberation" of Sunni Muslims in Shia-majority Iraq.
Most Indonesians are Sunni Muslims and tensions with minority Shias have intensified in recent years. Terrorism expert Andrie said returnees would likely exacerbate sectarian clashes.
Governments are increasingly concerned over the flow of foreigners to the Syria conflict, with the New York-based intelligence Soufan Group estimating some 12,000 have done so in the past three years.
Last month, an American man killed 37 people in a suicide bombing in Syria while a Belgian man who had fought with ISIL in Syria killed four people in a gun attack on a Jewish Museum in Brussels.
This week, Australia said around 150 Australians had learnt the "terrorist trade" fighting alongside Sunni militants in Iraq and Syria.
And in Britain the government has banned ISIL, with its security minister saying terrorism related to Syria's civil war "will pose a threat to the UK for the forseeable future".