Yemeni authorities have made more arrests of people suspected of involvement in the attack on a French oil tanker, which left one crew member dead.

It is believed that a small boat packed with explosives was used to attack the Limburg as it was off the Yemeni coast.

Investigators believe the tanker was rammed by a small boat

A manager and two employees at a marine supplies firm have now been detained, as well as two watchmen from a house allegedly used by the planners of the attack.

The total number of people arrested so far is about 20.

Earlier this month, Yemen's Prime Minister Abdul-Kader Bajammal said the attack on the Limburg might have been carried out by the same group who attacked the American warship, USS Cole, two years ago.

Yemeni authorities have tightened security around the country's ports following the attack, which sent more than 90,000 barrels of oil pouring into the Gulf of Aden.

A third of all international trade in oil passes Yemen's 2,400km (1,500 mile) coastline.

Boat theory

The theory that the Limburg was hit by a small boat packed with explosives was backed up by the earlier discovery of fragments from a small marine vessel on the deck of the tanker.

Yemeni officials believe the boat used in the attack came from outside Yemen, and was brought by land into the country.

French investigators found traces of TNT explosives on the tanker, providing the strongest evidence yet that the explosion was due to a terrorist attack.

Since the explosion, the French defence ministry has heightened security for French citizens in the Middle East, and has been considering military escorts for French commercial vessels in the region.

The style of the attack resembled the suicide bombing of the USS Cole in Yemen's Aden port in 2000, which killed 17 American sailors - an attack blamed on al-Qaeda militants.

Two statements last week attributed to Osama Bin Laden, the leader of al-Qaeda, praised the attack on the Limburg, but did not claim responsibility.