Speaking in Singapore amid a bloody standoff between Philippine troops and militants fighting under the IS 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," Ryacudu told the Shangri-La Dialogue, using another name for the IS group.
The threat of heightened terrorism, including the impending return of hundreds of Southeast Asian fighters who fought with IS in Syria and Iraq, has been a hot-button issue at the three-day Singapore summit also attended by US Defense Secretary Jim Mattis.
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.
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"How can we tackle these foreign fighters? We have to be comprehensive," said Ryacudu, 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."
"I really don't know, my figure is about 250-400, a lot less," he told reporters.
But David said there were 40 foreign IS 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," David said.
"That's why they were able to muster the operations in the area of Marawi," David said.