The Islamic State group's jihadist appeal is fanning fears that it could serve as a potent new rallying cry for Southeast Asian extremists who had been largely brought to heel following past deadly terror attacks.

Authorities in Indonesia -- the world's most populous Islamic country -- and Muslim-majority Malaysia have watched with alarm as scores, possibly even hundreds, of their nationals are believed to have gone to Syria and Iraq to join the fight for a hardline Muslim caliphate.

Terrorism analysts are increasingly concerned that these volunteers could import the organisation's violent ideology upon their return, or inspire supporters back home to carry out deadly attacks.

"There are still many breeding grounds for militancy in parts of the region, and if fighters come back they can strengthen these existing groups, and that's going to be a major problem," said Bantarto Bandoro of the Indonesian Defence University.

Already, the Philippine Islamist rebel group Abu Sayyaf has threatened to decapitate a German hostage taken earlier this year, recalling the grisly IS beheadings of foreign journalists and an aid worker that caused worldwide revulsion and triggered US air strikes.

Abu Sayyaf last week demanded a ransom and that Germany cease support for the strikes. The demands have been refused, with Manila dismissing the ultimatum as a cynical ploy to exploit the notoriety of IS for profit.

But the group's actions in Iraq and Syria are drawing troubling comparisons to the 1979-89 Soviet occupation of Afghanistan, which drew in volunteer Islamic fighters from around the world to fight the Communist "infidels", including from across Southeast Asia.

The hardened jihadis who returned nurtured a generation of Southeast Asian extremists, helping give rise to groups such as Jemaah Islamiyah (JI), which was responsible for deadly attacks including the 2002 Bali bombing that killed 202 people, mostly foreign tourists.

Aggressive Southeast Asian counter-terror efforts have since dramatically weakened JI and other militant groups, but they remain a threat.

Raising the stakes, the Islamic State group last week called on Muslims worldwide to kill citizens of the US anti-IS coalition and other "disbelievers."

Australian police a week earlier had thwarted a plot to take and kill hostages there, arresting 15 people.

Worried governments across the world are responding with various steps including banning IS, outlawing travel for jihadi reasons, or barring fighters seeking to return.

- Attacks on foreigners feared -

The Islamic State group's sophisticated utilisation of social media to broadcast its message globally has heightened the concerns in Southeast Asia, which has some of the world's highest rates of social media use.

The Jakarta-based Institute for Policy Analysis of Conflict (IPAC) warned last week that foreigners could again become targets in Indonesia, where some extremists have backed the IS exhortation to kill its enemies.

IPAC added that Indonesian and Malaysian fighters in Syria are believed to have formed their own group there, in a potentially ominous development.

"Members (of the group) could become the vanguard for a fighting force that would reach into Indonesia, Malaysia and the Philippines," it said in a policy report.

Joseph Chinyong Liow, an expert in Islamic militancy with Singapore's S. Rajaratnam School of International Studies, said counter-terror capabilities and intelligence in the region have increased dramatically since JI's heyday.

Today's militants have "lost the tactical advantage of surprise," he wrote in the latest issue of Foreign Affairs.

But authorities must remain vigilant as IS is ramping up recruitment online and through Southeast Asia's "local Islamic communities and networks, just as Afghan militants did in earlier decades," he added.

IPAC called for tighter prison management, noting that a leading Indonesian advocate of IS -- jailed radical cleric Aman Abdurrahman -- has been able to disseminate its propaganda from his maximum-security prison cell.

- 'We are monitoring' -

Authorities in Malaysia, which has historically kept a tighter lid on extremists, insist they are on top of the threat posed by IS, which has been declared an illegal terror group.

In August, police counter-terror official Ayob Khan Mydin told AFP that 19 people arrested in recent months were supporters of the group involved in an amateurish plot to carry out a wave of bombings in Malaysia.

Separately, police said three people recruited by the organisation on Facebook were arrested trying to leave Malaysia on Thursday.

"We are monitoring websites, Facebook and Twitter accounts," Ayob Khan told AFP, declining to give further details on police efforts.

"There is a high probability that those who are involved in (IS) will engage in militant activities once they return to Malaysia."

Experts said the predominantly Catholic Philippines could emerge as a greater source of concern due to its lower counter-terror capacities and weak hold on parts of its Muslim south.

Philippine officials insist they have no evidence any Filipinos had travelled to Syria or Iraq, but local officials in the south have warned of accelerating IS recruitment efforts there.

Bandoro said Islamic State fighters had "won the hearts" of Southeast Asian extremists and that authorities should meet to discuss the new regional security problem before IS networks across the region gain a footing and band together.

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