Armed pirates have raided a Singapore-owned oil tanker off the coast of Malaysia, taking three crew members and pumping millions of litres of diesel fuel over several hours.

Six pirates in a speedboat boarded the Naninwa Maru early on Wednesday off the coast of west Malaysia, Maritime Police Commander Abdul Aziz Yusof told the Reuters news agency. 

"We are doing a thorough investigation. We have ruled out kidnapping because no ransom demand has been made,'' said Abdul Rahim Abdullah, Malaysia's maritime police deputy commander.

We are doing a thorough investigation. We have ruled out kidnapping because no ransom demand has been made.

Abdul Rahim Abdullah, Malaysian Maritime Police Deputy Commander

He said the ship's Indonesian captain, chief engineer and a supporting crewmember were missing along with their passports and belongings. Authorities are investigating whether they were involved.

The pirates pumped out more than half the five million litres of diesel into two waiting vessels.

The tanker, travelling from Singapore to Myanmar, had Indonesian, Thai, Burmese and Indian crew members.

Possible syndicate

Regional security officials have previously said that pirates in the Malacca Strait may be part of a syndicate that can either have links to the crew on board target ships, or inside knowledge about the ship and cargo.

Attacks by gangs armed with guns and knives on shipping in the strait have ranged from 12 to 20 incidents a year over the last three years, says the Singapore headquarters of the Regional Co-operation Agreement on Combating Piracy and Armed Robbery against Ships in Asia.

Most of these incidents have involved the theft of ship's stores, cash and assaults on the crew.

That compares with a peak of 220 recorded attacks in 2000, according to the Kuala Lumpur-based International Maritime Bureau, which tracks pirate activity.

Source: Agencies