A Japanese-owned bulk carrier was boarded today in the fourth pirate attack in the Malacca Straits in just over a month, officials said, as Malaysia announced plans to place armed police on ships using the vital trading route.

The 26,014 tonne "Ocean Bridge" was attacked before dawn by pirates who robbed the ship's safe of its cash before fleeing by boat, maritime officials told AFP .

The attack took place at One Fathom Bank, a narrow area of the Strait just north of Malaysia's Port Klang at 3:00 am, said Noel Choong, head of the Piracy Reporting Centre of the International Maritime Bureau (IMB).

Choong said the IMB was particularly concerned over the latest attack because "it is very rare for pirates to hit ships there.

"Being a narrow part of the Straits, it could pose a navigational threat to other ships, especially if the pirates remove the crew and leave no one on the bridge," he said.

"It appears pirates are attacking ships close to Malaysian waters which are traditionally considered safe," he said, urging the Malaysian authorities to beef up security.

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.

The three nations last year began coordinated patrols in the strait, which is one of the world's top piracy blackspots.

Escort service

But with the recent increase in attacks, Malaysia has announced that it will also place armed and uniformed police officers on board tugboats and barges plying the waterway.

This is in addition to an escort service launched last month for tugboats and barges carrying valuable cargo, the New Straits Times reported.

"We have received several complaints from worried shippers and we have to assure them that the safety of their goods and the crew on the boats is being taken care of," said Internal Security and Public Order director Othman Talib.

"Those assigned will be in uniform and armed. We will station them on randomly chosen ships, the minute the vessels enter our waters," he said.

"They (pirates) will wait when there are no marine police boats nearby and strike at tugboats and barges which are alone and vulnerable," Othman said.

Friday's attack was the second on a Japanese-owned ship in a little over two weeks.

On March 14 a Japanese tugboat was boarded by armed men who kidnapped the captain and two crew members. They were freed a week later.

Floating bomb

Kidnappings at sea often lead to ransom demands, but it is not known whether money exchanged hands in that case.

On March 12, a gang of 35 pirates armed with machine guns and rocket launchers boarded an Indonesian gas tanker and kidnapped the captain and chief engineer. They were released after a ransom was paid.

On Feb 28, a Malaysian tugboat was attacked. The chief engineer was shot in the leg and the captain and chief officer were kidnapped, but later freed.

The ease with which pirates board ships in the strait has given rise to concerns in Japan and the West that terrorists could hijack a tanker to use as a floating bomb or to block the vital channel and disrupt world trade.