Southeast Asia is on high alert to prevent a repeat of last Friday's deadly attacks in Paris on November 13, as the Islamic State in Iraq and Syria (ISIS) threatens to launch more strikes.

In Manila, about 20,000 security and emergency services personnel have been deployed to secure the capital city, where no fewer than 17 heads of state are attending the two-day Asia-Pacific Economic Cooperation summit starting Wednesday.

Kuala Lumpur, which is hosting the 27th Asean Summit followed by the East Asia Summit this weekend, has mobilised more than 4,000 police officers at key locations.

Singapore and Indonesia are also taking no chances.

Defence Minister Ng Eng Hen said on Monday on November 16 that Singapore has stepped up vigilance at key installations, while Indonesia's counter- terrorism forces have tightened surveillance on citizens recently deported after trying to link up with ISIS.

Both countries' security forces are said to have ramped up border security as well as stepped up patrols, particularly in areas with foreign embassies or a lot of people.

On Tuesday, security at Juanda Airport in Surabaya city, East Java province, was reinforced by navy personnel. The Indonesian navy operates an air base at the airport.

Sources told The Straits Times recently that security alert levels in the region had been raised even before the Paris attacks, amid chatter of potential strikes by ISIS or its affiliates.

The threat from splinter groups in the region is very real, said experts, citing intelligence pointing to plans by militants in the southern Philippines to form a Southeast Asian faction of ISIS by bringing together terror groups such as Jemaah Islamiah and Abu Sayyaf.

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 figures also show that about 700 Southeast Asians, including a few Singaporeans, have travelled to the Middle East to fight with or receive training by ISIS.

Many have returned home, mostly deported from Turkey.

Inspector-General Arief Dharmawan, who is deputy head of enforcement at Indonesia's National Counter-Terrorism Agency, told The Straits Times that 162 Indonesians have been deported so far.

Among them were 40 men of "fighting age", who have since been released because there are no laws that allow for preventive detention in Indonesia, even though some had intended to join ISIS.

The threat from such fighters returning home is also a real concern for security agencies in Southeast Asia, said experts that The Straits Times spoke to yesterday.

Dr Damien Dominic Cheong, from the Centre of Excellence for National Security at the S. Rajaratnam School of International Studies, said some of these men could be dispatched to carry out attacks or conduct other operational activities like recruitment.

"In any case, such returnees have increased 'street cred' from being in Iraq or Syria and, in some cases, fighting for ISIS. As such, they could be used for both radicalisation and counter-radicalisation purposes," said Dr Cheong.

"For the former, they would instantly attract followers and sympathisers, thereby making the establishment of cells or groups easier."

Security consultant Susan Sim, from the New York-based The Soufan Group, agrees, adding that these are seasoned fighters who learnt their tradecraft in Afghanistan and Mindanao, and may be craving yet another battle.

These are reasons why attacks similar to what happened in Paris can happen in Southeast Asian cities such as Singapore, said Dr Cheong. "Their tactics would probably be different, but the impact would be just as devastating."

As for the risk of individuals returning from the Middle East undetected, he said international cooperation between security agencies and good local intelligence can help mitigate this.

"It's the guys you don't know about that keep policymakers up at night," said Sim.

• Additional reporting by Eunice Au in Kuala Lumpur and Raul Dancel in Manila