The U.S. military believes at least 1,000 jihadist fighters have been inspired to leave their homes in Asia to fight with militant groups across the battlefields of Iraq and Syria.

“Our estimations today is there’s probably been about 1,000 potential aspiring fighters that have moved from this region, based on kind of our overall assessment,” Admiral Samuel Locklear, the U.S. Pacific Command commander, told reporters at the Pentagon on Thursday.

“That number could get larger as we go forward, but certainly that’s about the size or the magnitude that we perceive at this point in time.”

The Asia-Pacific is currently home to myriad homegrown jihadist networks, from restive enclaves in the Philippines and Indonesia to the rough tribal highlands of Pakistan and Afghanistan.

Authorities in the region have long grappled with combating Muslim extremists, who travel abroad to participate in Islamist terrorist networks, only to return and wreak havoc on the home front later.

During the Soviet Union’s occupation of Afghanistan in the 1980s, an estimated 800 fighters from across Southeast Asia and Australia joined the mujahedin’s ranks battling the Red Army.

The militants who survived and returned to their respective countries went on to form the core of several Islamist extremist terrorists groups that orchestrated attacks across the region, including the bombing of nightclubs in Bali in 2002 and the Australian Embassy in Jakarta two years later.

“All these attacks, the masterminds were Afghan veterans,” Rohan Gunaratna, head of the International Centre for Political Violence and Terrorism Research at Nanyang Technological University in Singapore, tells TIME.

Experts fear that the new battlegrounds in the Middle East will provide the latest and larger crop of jihadists from the Asia-Pacific with the operational knowledge and connections to conduct larger attacks at home in the future.

“They will come back with motivation, ideology and skills and operational knowledge,” says Gunaratna. “They will know who should they contact in order to plan and execute an operation.”

And according to Gunaratna, the Islamic State of Iraq and Greater Syria (ISIS) appears to be winning the hearts and minds of aspiring jihadists across the continent, thanks to their slick propaganda films and robust social-media campaigns, as “opposed to the boring lectures delivered by al-Qaeda and Taliban ideologues.”

“It’s a new level of strategic communication that is being started by ISIS,” says Gunaratna.

However, experts admit the difficulty in tracking whom fighters align themselves with once they’ve made it to the Middle East.

“Once they cross the border it’s hard to tell who is with who,” says Rodger Shanahan, a nonresident fellow at Australia’s Lowy Institute for International Policy, by email.

But outside of just convincing fighters to move abroad, ISIS’s message appears to be motivating extremists to take action locally as well.

Earlier this week in the Philippines, terrorist group Abu Sayyaf, which pledged allegiance to ISIS this summer, threatened to kill two German hostages unless Berlin backs out of a U.S.-led coalition that began striking militant positions in Syria this week.

“The participation with support from Germany to America must stop, in the killing of our Muslims brothers in Iraq and Sham [Greater Syria] in general, and the mujahedeen of the Islamic State in particular,” read a translation provided by SITE Intelligence Group published by the Long War Journal.

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