Southeast Asia is grappling with rampant maritime piracy that, if left unchecked, could threaten trade and bolster terror networks in the region.

This year has seen a growing number of hijackings in the Sulu Sea, located between the Philippines and the island of Borneo. Around $40 billion worth of cargo passes through the area annually, with around $700-800 million in Indonesian coal exports going to the Philippines.

Philippine marines keep a look out for pirates in the Sulu Sea. H. Christoph / ullstein bild / Getty Images

Two German citizens were attacked—one was abducted and the other killed—earlier this month while sailing their yacht near Sabah, a Malaysian state in Borneo. And over the weekend, two Indonesian fishermen were abducted in two separate incidents off Sabah. In October, a South Korean ship was seized, with its captain and Philippine crewmember also taken hostage. Militant group Abu Sayyaf is believed to be responsible for the violence, according to the Philippine military. Based in the southern Philippine province of Mindanao, Abu Sayyaf has increasingly taken to maritime kidnappings for ransom payments, which brought in nearly $7.3 million so far this year, the Associated Press reported last month, citing a Philippine government report. One of Abu Sayyaf's goals is to establish an independent state based on Shariah law in Mindanao and preserve its ethnic Moro community. The onslaughts are a mixture of opportunistic and calculated crime, explained Zachary Abuza, a professor specializing in Southeast Asian security issues at the National War College. Abu Sayyaf can score a quick payoff through kidnappings and use the funds to finance broader terror operations, he said. That modus operandi is symbolic of the wider trends in global piracy. A report by IHS Markit earlier this month found that while the number of piracy attacks around the world have generally decreased, kidnapping had become more common. The seas around the Philippines were the most pirated, followed by Nigeria and India, the report said.