Indonesia should boost security in the Malacca Strait, an international maritime watchdog said today after pirates attacked a Japanese carrier and two UN ships off the coast of Aceh.

"We urge Indonesian authorities to take action to contain piracy in the strait," the head of the International Maritime Bureau's (IMB) Piracy Reporting Centre, Noel Choong, told AFP .

Choong said piracy attacks had declined in the Malacca Strait dividing Malaysia and Singapore after Indonesia launched agressive patrols codenamed Operation Gurita.

In the latest incident yesterday, pirates attacked the Japanese operated bulk carrier MV Island Oasis in waters off tsunami-hit Aceh.

The 20 Filipino crew prevented the pirates, believed to be Indonesian, from boarding the ship.

UN-chartered ships attacked

In the same area off the coast of Aceh in the north of the strait, pirates attacked two UN-chartered ships hours apart on Sunday.

In the first attack, some 12 pirates armed with AK47 rifles, hand grenades and pistols boarded the ship.

Both ships were ferrying construction materials to Indonesia for the UN's World Food Programme when they were attacked under cover of darkness.

Choong said seafarers must remain alert when plying the strait which is one of the world's most important waterways.

Maritime officials have said that the high cost of fuel is one key element restricting Indonesia's ability to conduct frequent maritime patrols along its vast coastline.

Leading insurer Lloyd's has placed the Malacca Strait on its list of dangerous waterways.