``Pirates in seven small fishing boats surrounded the tanker and attempted to board it late Tuesday,'' said Noel Choong, head of the International Maritime Bureau's Kuala Lumpur-based Piracy Reporting Center.

The 150,000-tonne Yohteisan, the length of a football field, was headed east just past the southern tip of the Malacca Strait in heavy rain and poor visibility when the incident occurred near Indonesia's Karimun islands.

Choong said the captain's tactics to shake off the pirates included increasing speed and added that all crew members are safe and the tanker has continued with its journey.

He said the tanker could have posed an environmental disaster if the pirates had succeeded in boarding the ship and taken over control.

``Anything could have happened,'' he said. ``Singapore, of course, will take this attack seriously.''

Another maritime official said tankers the size of the Yohteisan would be ideal for militants to use to block choke points in the narrow Malacca Strait or in the Singapore Strait.

That scenario has been advanced several times recently.

A Japanese-owned bulk carrier was boarded last week - the fourth pirate attack in the Malacca Strait in just over a month.

The 26,014-tonne Ocean Bridge was attacked before dawn Friday by pirates armed with firearms and knives. They robbed the ship's safe of its cash before fleeing on their boat.

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.

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

The 960-kilometer-long Malacca Strait, 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. But with the recent increase in attacks, Malaysia has announced it will place armed and uniformed police officers on board tugboats and barges plying the waterway.

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