Heavily-armed pirates have attacked two Singapore-registered tugs in Indonesian waters of the Malacca Straits and kidnapped four crew, a maritime official said today.

"In the last five days, we had two incidents of pirate attacks off Belawan in Indonesia. They have kidnapped the captain and chief engineers of the two boats," Noel Choong, regional manager of the International Maritime Bureau (IMB) in Kuala Lumpur, told AFP .

In the first incident on Sept 30, armed robbers, believed to be Indonesians, boarded a tug and seized telephones and ship's documents.

"They then kidnapped the captain and the chief engineer. Until now we do not know their fate or where they are being detained," Choong said.

On Saturday, around eight heavily-armed pirates in a fast boat fired at a tug before boarding it.

Attacked off Port Klang

Choong said the pirates, believed to be the same gang that carried out the first attack, again kidnapped the captain and chief engineer and stole valuables from the 16-man crew of mainly Filipinos and Thais.

"The crew onboard are in a state of shock," he said.

Choong said the tug proceeded to Singapore after the attack, only to be attacked by more pirates off Port Klang.

"The boat and the crew have sought refuge in the port," he said.

Choong said it was likely the pirates would contact the ship owners for ransom. There have been seven cases of kidnap and ransom in the Malacca Straits since May, he said, adding that the IMB would write to Indonesian authorities to beef up patrols off Belawan.

Indonesia, Malaysia and Singapore have began year-round coordinated naval patrols in the pirate-infested Strait.