SINGAPORE (AFP, BLOOMBERG) - There are about 1,200 ISIS operatives in the Philippines, including foreigners of whom 40 are from Indonesia, the Indonesian defence minister told an international security forum on Sunday (June 4).

General Ryamizard Ryacudu also estimated there were as many as a million ISIS sympathisers in Indonesia, the world's most populous Muslim-majority nation, but fewer than 700 were a "real problem."

The broader South-east Asian region is home to about 15 per cent of the world's 1.57 billion Muslims.

Speaking in Singapore amid a bloody standoff between Philippine troops and militants fighting under the Islamic State in Iraq and Syria (ISIS) group flag in Marawi city, Defence Minister Ryamizard Ryacudu called the militants "killing machines" and urged full-scale regional cooperation against them.

"I was advised last night, 1,200 ISIS in the Philippines, around 40 from Indonesia," General Ryacudu told the IISS Shangri-La Dialogue, organised by the London-based International Institute for Strategic Studies.

"The terrorism threat in this region has evolved into an unprecedented immediate level of emergency," said Gen Ryamizard, whose capital, Jakarta, was struck last month by twin suicide attacks that killed three police officers.

"The death group's area of operation has gone global."

He called for expanding anti-piracy patrols initiated in August by Indonesia, Malaysia and the Philippines near Mindanao to include Singapore and Thailand and focus on terrorism. President Joko Widodo has called on lawmakers to expedite the passage of revised anti-terrorism laws to give police more power.

The threat of heightened terrorism, including the impending return of hundreds of South-east Asian fighters who fought with ISIS in Syria and Iraq, has been a hot-button issue at the three-day Singapore summit also attended by US Defence Secretary James Mattis.

Hundreds of Islamist gunmen rampaged through Marawi, a largely Muslim city of 200,000 in the south of the mainly Catholic Philippines, on May 23 after government forces attempted to arrest their leader, Isnilon Hapilon.

Up to 50 gunmen are still controlling the city centre nearly two weeks after the start of fighting that has killed 177 people including 120 militants.

"How can we tackle these foreign fighters? We have to be comprehensive," said Gen Ryamizard, a retired general.

"We have to find... complete ways but we must exercise caution, they are killing machines. Their aim is to kill other people so that's why it's our responsibility that we have common understanding, consensus and common proceedings on how to fight these foreign fighters."

Philippine Defence Undersecretary Ricardo David, speaking at the same forum, said the 1,200 figure for total ISIS fighters in the Philippines mentioned by Indonesia was new to him.

"I really don't know, my figure is about 250-400, a lot less," he told reporters.

But General David said there were 40 foreign ISIS fighters among those who seized parts of Marawi, eight of whom have been killed by government forces.

Earlier, Philippine officials said the slain foreign fighters were from Malaysia, Indonesia, Yemen, Saudi Arabia and Chechnya.

"Our intelligence estimates that there are about 40 foreigners that fought in the Marawi incident," Gen David said.

The Philippine official added that the foreign fighters used "back channels" in the Sulu and Celebes Seas near the borders of the Philippines, Indonesia and Malaysia to enter the southern island of Mindanao and link up with local terror groups.

"That's why they were able to muster the operations in the area of Marawi," Gen David said.