The armed forces of Indonesia and Malaysia should strengthen security measures, Singapore urged. Malaysia's coastguard has already stepped up security in the narrow waterway, which tankers use to carry oil from the Middle East to China and Japan.
"We will increase security and step up patrols in that area. Oil tankers can pass, but we will increase our readiness," Indonesian Defence Minister Purnomo Yusgiantoro said.
An attack that closed the Strait of Malacca or Singapore port even temporarily could have a disproportionate impact on global trade.
“Maritime attacks offer terrorists an alternate means of causing mass economic destabilisation," Rand Corporation terrorism risk analyst Peter Chalk said.
The city-state of Singapore is the world's top container shipping port and biggest ship refuelling hub.
The effects of an attack remain uncertain. Thailand’s naval attaché in Singapore said the original warning came from Japan. “Later, they changed the term to `terrorist attack' as there was fear that they could use heavy weapons to attack these ships,” Captain Sutheepong Kaewtab said.