The attack occurred on April 17 when 16 pirates in a speedboat boarded the ship under the cover of darkness, said head of the IMB's Kuala Lumpur-based Piracy Reporting Centre Noel Choong.
"The tanker was hijacked off Aur Island, in the eastern coast of Malaysia. The heavily-armed pirates hit the captain with a sword and destroyed communication equipment," he told AFP.
Choong did not have any more details on the condition of the captain who oversaw 14 crew members on the tanker en route from Singapore to Cambodia.
"The pirates siphoned part of the tanker's diesel cargo into a tanker ship and stole crew members' valuables before escaping," he said, adding that the IMB was only informed of the incident on Thursday due to the ship's damaged communications system.
Choong said the IMB was concerned about the rise in attacks off Malaysia this year.
On Tuesday around 12 heavily-armed pirates boarded a Singapore-managed tanker in the Strait of Malacca, kidnapping three Indonesian crew and stealing some of the diesel fuel from the cargo.
The Strait of Malacca is a key maritime highway linking Europe and the Middle East to Asia, and has long been a hunting ground for pirates.
Attacks in the strait had dropped in recent years following stepped-up patrols and cooperation between neighbouring countries to secure waterways.
There were no pirate attacks in Malaysian waters last year and Tuesday's attack in the Strait of Malacca was the first since 2010, according to IMB data.(AFP)
