Indonesian separatist rebels may have carried out the armed attack on a Japanese tugboat in the Malacca Straits and the kidnapping of three crewmen, Malaysia's marine police said today.

"The pirates are from Indonesia. They may be GAM rebels. I will not rule it out," Mokhtar Othman, operations officer for Malaysia's northern region marine police, told AFP .

Mokhtar said the fact the pirates were well armed and "had something like a rocket launcher" pointed to the involvement of Free Aceh Movement rebels, who since 1976 have been waging a separatist revolt in the north of Sumatra island.

GAM spokesman Sofyan Daud denied that the rebels were behind the attack, saying they had never been involved in piracy in the Malacca Straits.

"We are not involved in activities outside Aceh, let alone in piracy. We are freedom fighters, not pirates," Daud told AFP by telephone.

The London-based International Maritime Bureau (IMB) has in the past suggested that GAM rebels could be involved in piracy and warned ships to stay clear of Aceh's coast.

The tugboat was on its way from Indonesia's Batam island to Burma on Monday when the pirates kidnapped 56-year-old captain Nobuo Inoue, 50-year-old chief engineer Shunji Kuroda and a Filipino crewman identified by Manila as Sangdang Paliawan, 31.

The tug with its 11 remaining crewmen and the 154 workers on the construction barge it was towing were escorted to Malaysia's Penang island by marine police.

Mokhtar said the crew, the tugboat and barge would be released in the next few days when investigations were completed.

The pirates, who are suspected of planning to make a ransom demand, had not yet communicated with the Japanese ship owners, he said.

Fate of crew unknown

Penang police chief Christopher Wan Soo Kee said the fate and location of the kidnapped crew were still unknown.

"There is no new development. Our marine police are still patrolling the area. Together with the Indonesian police, we are trying to work closely to identify any of the hideouts," he told AFP .

The attack came as Malaysia was conducting an eight-day air-sea exercise some 100 nautical miles away in the Malacca Straits involving six boats, two aircraft and 100 men. The exercise ended Wednesday.

In an earlier raid in the strait at the weekend, a gang of 35 pirates armed with machine guns and rocket launchers boarded the Indonesian-registered Tri Samudra, which was carrying 1,300 tonnes of methanol, and kidnapped the captain and the chief engineer.

The kidnappers have already demanded a ransom of two billion rupiah (US$214,000) from the boat owner, Humpuss Intermoda Transport. The ship's owner has said GAM rebels were suspected of involvement.

Captain Pottengal Mukundan, director of the London-based IMB, said in a statement: "The recent upsurge of violence against vessels in the Malacca Strait is a matter of great concern.

"It is vital that action be taken by law enforcement agencies to identify the perpetrators of these attacks and have them punished under law.

"The abduction of crew members in this manner is a reprehensible act and should not be allowed to continue."

The narrow 960-kilometre-long Malacca Straits, bordered by Malaysia, Singapore and Indonesia, is used by about 50,000 ships a year carrying a third of world trade and half its oil supplies.