A gang of pirates attempted to board a huge Japanese-owned crude oil tanker in the Singapore Strait in the latest of a series of attacks on the region's strategically important shipping lanes, a maritime official said today.

"Pirates in seven small fishing boats surrounded the tanker and attempted to board it late Tuesday," Noel Choong, head of the Piracy Reporting Centre of the International Maritime Bureau (IMB) told AFP .

The 150,000-tonne 'Yohteisan' was on an east-bound journey in heavy rain and poor visibility when the incident occurred off Indonesia's Karimun islands, where the southern tip of the Malacca Straits joins the Singapore Strait.

Choong said the captain of the Panama-registered tanker took evasive measures and increased speed to escape the attackers.

All the crew members were safe and the tanker continued on its journey, he said, adding that a successful boarding of a tanker of this size in the narrow sea lane could have had disastrous consequences.

"Anything could have happened. Singapore of course will take this attack seriously," Choong said.

Pirate-infested straits

The foiled raid comes after four pirate attacks on ships in the Malacca Straits in the past six weeks, which have added to concerns that armed extremists could hijack a tanker to use as a floating bomb or to block the vital channel and disrupt world trade.

On April 1, the 26,014 tonne Japanese-owned bulk carrier 'Ocean Bridge' was attacked before dawn by pirates armed with guns and knives who robbed the ship's safe of its cash before fleeing.

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.

Two days earlier 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 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.

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

Malaysia has, however, rejected suggestions that the United States or other foreign navies be allowed to help patrol the strait.