Afief Abdul Madjid was given the cellphone number of a man in Turkey, whom he had never met, named Kholid.

"Call Kholid once you arrive in Turkey. He will take you across to Syria and to ISIS," a local fixer for the Islamic State in Iraq and Syria (ISIS) militant group told Afief.

That was just over two years ago in Yogyakarta, Indonesia.

Afief, then a cleric, and a friend named Agung indeed made their way to Turkey, where they linked up with Kholid in Hatay province near the Syrian border.

From Hatay, the two Indonesians were smuggled across to Syria and taken to an ISIS fighters' training camp in Ladzikiyah, where they learnt, among other things, how to handle weapons.

Counter-terrorism hotline for public

Members of the public can play their part by remaining vigilant and reporting suspicious activity, including suspicious online activity, to the authorities. Similarly, if a person knows of someone who is radicalised and may pose a security threat, he should alert the authorities. The public can contact the police on 999, or the Inter-nal Security Department Counter-Terrorism Centre on 1800-2626-473 (1800-2626- ISD).

After completing the training in January last year, Agung stayed on to fight in Syria, while Afief returned to Indonesia, where he promptly visited Abu Bakar Bashir, the spiritual leader of the Jemaah Islamiah (JI) terror group, in prison.

Afief was arrested shortly after in West Java, and has since been jailed for four years. His ISIS fixer, said to be a small-town doctor in Yogyakarta, however, remains at large and possibly free to send more ISIS aspirants into the fray.

Afief's story is not an uncommon narrative among extremists who travel to conflict zones overseas to receive training from groups like ISIS or Al-Qaeda, and then return to plot or support terror activities in their homeland.

Similar reports have been recorded by security and intelligence agencies throughout South-east Asia, from as far back as the 1980s, after the Soviet-Afghan war.

At the time, sympathisers from South-east Asia who had left home to fight alongside the mujahideen against the Soviets in Afghanistan or Pakistan returned and set up militant groups like JI.

JI, which was responsible for the Bali bombings that killed 202 people in 2002, ran a wide network of terrorist cells in the region at its prime, including one in Singapore led by Mas Selamat Kastari, who is now in custody.

A wave of arrests, however, largely neutralised the group in the years following the Sept 11, 2001, attacks in the United States. But now, ISIS militants returning home - either deported by the Turkish authorities or travelling undetected across borders - may spark violence reminiscent of the JI threat of old.

They could possibly plan attacks similar to those that happened in Paris in this part of the world, said security experts and analysts.

Terror plots in this region, however, are believed to have been in motion even before the incidents in Paris or Bamako in Mali - possibly with ISIS-trained fighters.

Various sources have told The Sunday Times that security alert levels in South-east Asian countries had been raised even before the Paris attacks, amid chatter of potential strikes by ISIS or its affiliates in the region.

Security measures were also ramped up further in Manila - which hosted the two-day Asia-Pacific Economic Cooperation summit that ended last Thursday - and Kuala Lumpur, where the Asean and East Asia summits are being held. This, as the subject of terrorism hogged the agenda during the regional meetings.

A leaked memo issued by Malaysian police last Monday also revealed that the Philippine militant group Abu Sayyaf and ISIS had deployed 10 suicide bombers in Kuala Lumpur and eight in Sabah who "underwent military training in Syria, Afghanistan and Iraq".

Indonesia's National Counter- Terrorism Agency (BNPT) chief Saud Usman Nasution warned that his countrymen fighting for ISIS could return and supply training, funds and organisational skills to local militant groups.

The BNPT said 162 Indonesians have been deported from Turkey so far. All have been released because there are no laws that allow for preventive detention in Indonesia, even though some had intended to join ISIS, but 46 among them who are men of "fighting age" remained under surveillance by the counter- terrorism forces.

The threat from splinter groups of Middle Eastern terrorist organisations in the region is very real, said experts, citing intelligence pointing to plans by militants in the southern Philippines to form an ISIS faction, by bringing together terror groups such as JI and Abu Sayyaf, to launch operations in South-east Asia.

Already, as many as 30 militant elements from Indonesia, Malaysia and the Philippines are believed to have pledged allegiance to ISIS in the past year - evidence of the group's growing influence in the region.

Latest estimates from security agencies in Indonesia, Malaysia, Singapore and the Philippines show at least 900 from all four countries have travelled to join ISIS in Syria.

The largest number, about 700 of them, are from Indonesia, home to the world's largest Muslim population, while Malaysia and the Philippines each has about 100 citizens suspected to have made the journey. By comparison, France, in the aftermath of the siege on Paris which saw 130 killed, estimates that 400 of its citizens have done the same.

The sheer number of people coming from the three Asean countries has led to the formation within ISIS of a South-east Asian unit called the Katibah Nusantara (KN), which is led by former JI member Bahrum Syah.

Associate research fellow V. Arianti and senior analyst Jasminder Singh from the S. Rajaratnam School of International Studies (RSIS) said several KN fighters have aided terrorist groups in Indonesia.

"More terrorist cells in Indonesia might gain financial and logistical assistance from their comrades in Syria," they said in a study out last week.

"The growing links between Indonesian extremists and ISIS might give rise to more variation of targets for terrorists... Additionally, foreign embassies might once again become terrorist targets."

A former case officer with the Central Intelligence Agency of the US, Mr Patrick Skinner, said the spectre of direct command and control - in terms of ISIS' reach - is worrisome.

"There are 30,000 foreign fighters who have gone to fight for the Islamic State; even a small percentage of those able to return, such as several of the Paris attackers, represents a clear and present danger," said the director of special projects at New York-based security consulting firm The Soufan Group.

He added that it is much harder, from a counter-terrorism perspective, to disrupt ill-formed cells of local family members and friends than it is to detect and disrupt plots that travel along traditional centralised terror nodes of command and control.

"Small yet effective terror cells that the group has hardened and shaped - but only loosely directed - represent a true counter-terrorism nightmare," said Mr Skinner.

Singapore's Ministry of Home Affairs said that although there is no information of a specific terrorist threat to Singapore at present, the terrorism threat remains persistent.

"An attack can take different forms, including 'lone wolf' terrorist attacks which are difficult to detect," a spokesman said yesterday.

"As seen in the experiences of other countries, an attack could still occur even with heightened security."

•Additional reporting by Wahyudi Soeriaatmadja, Amy Chew and Raul Dancel