Singapore has issued a warning that an unnamed group may be planning to attack oil tankers in the Malacca Strait, one of the world's busiest shipping lanes.
Other large vessels with dangerous cargoes were also at risk.
The Navy said the group's intent was probably to achieve widespread publicity and show that they remain a viable group.
The navy recommended ships strengthen their onboard security measures, remain vigilant and report unusual incidents.
Successful attacks on tankers have been carried out using small vessels such as dinghies, speedboats and fishing boats, which are common in the Malacca Strait.
The BBC reports about 33% of global seaborne crude oil shipments pass through the 965km channel - which is shared by Malaysia, Indonesia and Singapore.
The seaway is almost six times busier than the Suez Canal.