MANILA/KUALA LUMPUR (REUTERS) - South-east Asian countries fighting Islamic State in Iraq and Syria's (ISIS) influence in the region have lauded the killing of its leader Abu Bakr al-Baghdadi but they said security forces were preparing for a long battle to thwart the militant group's ideology.

The Philippines, Indonesia and Malaysia, home to some of Asia's most organised Islamist militants, said on Monday (Oct 28) they were braced for retaliation by ISIS loyalists, including "lone wolf" attacks by locals radicalised by the group's powerful online propaganda.

Baghdadi killed himself in a tunnel in north-west Syria by detonating a suicide vest as US forces closed in, according to US President Donald Trump.

Though his death will unsettle ISIS, it remains capable and dangerous, said Delfin Lorenzana, defence secretary of the Philippines, where the group's influence has taken a hold among unschooled Muslim youth in its troubled Mindanao region.

"This is a blow to the organisation considering al-Baghdadi's stature as a leader. But this is just a momentary setback considering the depth and reach of the organisation worldwide," Lorenzana said. "Somebody will take his place."

South-east Asia has long been an important focus for ISIS, which has inspired Islamist militants in West Africa, across the Middle East and Asia and through to Indonesia and the Philippines.

The Philippines, Malaysia and Indonesia are concerned ISIS supporters from the region and those fleeing Iraq and Syria could exploit the porous borders, lawlessness and abundant arms found in Mindanao to take refuge in its far-flung villages.

ISIS has claimed responsibility for four suicide bombings since July last year in the Philippines, which fought its toughest battle since World War II in 2017 when extremists seeking to establish an ISIS laid siege to Marawi City and occupied it through five months of air and ground assaults.

Fighters from at least seven countries took part, including Malaysia, which remains on high alert and has arrested 400 people suspected of links to militant groups.

Malaysian police counter-terrorism chief Ayob Khan Mydin Pitchay said the real concern was not ISIS's leadership but the effect of its teachings.

"It's good news, but his death will have little impact here as the main problem remains the spread of the Islamic State ideology," he told Reuters.

"What we are most worried about now are 'lone wolf' attacks and those who are self-radicalised through the Internet. We are still seeing the spread of ISIS teachings online. ISIS publications and magazines from years ago are being reproduced and re-shared," he said.

'MANY-HEADED MONSTER'

Chatrooms in messaging applications used by Islamists such as Telegram showed defiant messages about Baghdadi's death, according to a researcher who monitors activity by ISIS sympathisers.

"God willing, whatever happens, Islamic jihad will not rely on any one individual, but will always stand tall on the orders of God and His Prophet," read one posting under the handle Ansurul Ummah.

Another participant, Abu Abdullah Asy Syami, posted: "Jihad will never stop, even if our own caliph dies."

Australian Prime Minister Scott Morrison made a similar observation, and said Baghdadi's death was by no means the end.

"This is a many-headed monster ... As you cut one off, another one inevitably arises," he told reporters.

Indonesia, the world's biggest Muslim-majority country, is grappling with a resurgence in militancy and has detained hundreds of suspects this year under tightened anti-terrorism laws.

Authorities believe thousands of Indonesians draw inspiration from ISIS and about 500 are thought to have joined the group in Syria.

Indonesia's intelligence agency said it was ready for retaliation and though Baghdadi's death would be a psychological blow, ISIS would have a successor in place.

"It is a war. Usually, there must be a counterattack or the like. When it comes to security, we are sure that we will secure this country," said its spokesman, Wawan Purwanto.

Security analyst Rommel Banlaoi said Baghdadi's demise and uncertainty about the leadership could undermine operations of ISIS loyalists seeking to regroup and establish their own territory in South-east Asia.

"Pro-ISIS groups in the Philippines will surely re-examine their roles in the post-Baghdadi era," he said.