Ng was speaking to reporters following a meeting of Southeast Asia defense ministers with US Defense Secretary Ash Carter in Hawaii, where the defence chiefs devoted a major chunk of time to discussing the threat from ISIS and similar groups.
Over 1,000 Southeast Asians have flocked to join ISIS's self-declared "caliphate" in Iraq and Syria, Ng said.
"Every year we meet, the situation and threat from extremist terrorism rises," Ng said. "Compared to, say, a year or even two years ago, they're more organized...they're more networked, they're more clear in their articulation of what they want to achieve."
Security officials said in June that Southeast Asian militants who claim to be fighting for ISIS said they had chosen one of the most wanted men in the Philippines to head a regional faction of the group.
Authorities in the region have been on heightened alert since ISIS claimed an attack in the Indonesian capital Jakarta in January in which eight people were killed, including four of the attackers.
Ng cited a recent plot in which an Indonesian cell coordinated by an ISIS member in Syria had planned to shoot up hotels in Singapore's Marina Bay area. The plot was foiled by Indonesian authorities, he said.
US defence officials believe that hundreds of citizens from Southeast Asia have returned to their home countries after joining Islamic State in Iraq and Syria, raising concerns that they may try to launch a wave of attacks.
Ng said Southeast Asian defence chiefs believe that if ISIS were destroyed in its base in Iraq and Syria, that would actually increase the risk to their region in the short-term, because seasoned ISIS terrorists would likely return home. The United States is leading an air campaign to dislodge ISIS from Iraq and Syria.
"They would decide to come back, some re-energized, some trained and the networks would still be existing," Ng said.
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