Choong said both ships were ferrying construction materials to Indonesia for the UN's World Food Programme when pirates boarded the ships under the cover of darkness on Sunday.

It was the fifth attack so far this year and the IMB would issue an alert if more attacks take place in the Malacca Strait, which runs between Indonesia and Malaysia, he said.

In the first incident, pirates believed to be Indonesians attacked a UN ship with a 10-strong crew sailing from Belawan to Lhokseumawe on Aceh's eastern shores, stealing some items and damaging the boat's equipment.

Some three hours later, another UN ship carrying a similar humanitarian mission sailing from Belawan to Calang on the west coast of Aceh was attacked.

"Pirates stole cash and some construction materials," Choong said.

"It could be the same group. The attacks happened hours apart."

Choong urged ships to maintain anti-piracy watches and pressed the Indonesian authorities to step up patrols to prevent new attacks.

"We hope the two attacks are isolated incidents and not a start of more attacks," he said.

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.

Three states bordering the strategic waterway -- Malaysia, Indonesia and Singapore -- had launched aggressive joint patrols.

A history of piracy and kidnapping of sailors prompted the three countries to launch joint sea and air patrols aimed at securing one of the world's most important waterways.

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

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