Pirates briefly seized a chemical tanker in the Malacca Strait, releasing the boat and crew but holding the captain and chief engineer for ransom. The latest act of piracy in one of the world's busiest shipping lanes underscores security concerns for that region.

Around 35 pirates armed with machine guns and rocket launchers seized the Indonesian-owned chemical tanker on Saturday before disembarking with the ship's captain and chief engineer.

The attack took place as the ship was sailing from Kalimantan on Borneo Island for the Indonesian port of Belawan in the Malacca Strait, one the world's busiest sea ways.

Almost all of China and Japan's oil imports and more than a quarter of global trade pass through the Strait, an area plagued by pirates.

Noel Choong of the International Maritime Bureau, a non-profit shipping association in Malaysia, says the ship was carrying an unknown but highly flammable liquid.

Mr. Choong says the incident was simply a criminal act, not terrorism. He says that on Monday the owners of the vessel are negotiating with the pirates for the release of the captain and engineer.

"At the moment we can safely say that it's a piratical attack, it's not for political gain, but it's for monetary [gain]," said Noel Choong. "But of course a lot of people are also afraid that terrorists may actually learn from the pirates."

Mr. Choong says it is impossible to say who is behind the attack, although the ship's owners blame Indonesia's Aceh separatist group, the Free Aceh Movement, or GAM.

But Mr. Choong says pirates often attack ships in the Strait and blame GAM.

The attack underscores international concerns that terrorists may team up with pirates and launch an attack in the Malacca Strait or block the waterway to disrupt world trade. One particular concern is that terrorists would seize a tanker carrying oil or gas, and then crash it into a port city.

The narrow 960-kilometer Malacca Strait is bordered by Malaysia, Indonesia, and Singapore. Last year, the three nations began coordinated security patrols in the waterway.

Security experts say the type of ship seized Saturday is the perfect size to use as a weapon.

Increased security has made land targets more difficult for terrorists, so some groups are looking for maritime targets. On Friday Malaysia said it would boost security in the Malacca Strait with a 24-hour radar system.