(Adds shipping executives' comments, changes dateline, byline)

By Rawhi Abeidoh

MUKALLA, Yemen, Oct 13 (Reuters) - Yemen is now convinced attackers set off the blast that gutted a French supertanker in the Gulf of Aden last week and Western shipping executives said on Sunday the assault was probably the work of suicide bombers.

Yemen, trying to shed an image as a haven for Islamist militants, had initially said a fire caused the explosion which killed one crewman, but sources close to the government-led probe said the Arab state was now sure it was deliberate.

French diplomats, the Limburg's crew and its owner, Euronav SA, had said from the start last Sunday's blast had probably occurred after a small boat rammed into the tanker's hull.

"Yemen is now convinced that it (the explosion) was a pre-meditated act but the question remains of who did it," one of the Yemeni sources said on Sunday.

Yemen, which has arrested more than 100 suspected members of Osama bin Laden's al Qaeda network and other Islamist groups in the past year, said it had rounded up about 20 people as a "pre-emptive measure" since the Limburg explosion.

The French-flagged tanker, carrying 400,000 barrels of Saudi crude, was waiting to be tugged into Mina al-Dabah port near Mukalla when an explosion ignited a fire on board and gouged a huge hole in its hull.

Experts said the blast bore resemblances to the October 2000 attack on the U.S. destroyer Cole in Aden which Washington blames on al Qaeda. In that attack, suicide bombers rammed an explosives-laden boat into the Cole, killing 17 U.S. servicemen.

Last month, the U.S. Navy warned of possible al Qaeda attacks on oil tankers in the Red Sea and Gulf which carry almost one third of the world's crude production.

BLAST HIT ONLY FULL TANK

On Friday, French Defence Minister Michele Alliot-Marie said traces of TNT had been found inside the tanker, a day after U.S. and French experts found fibreglass debris from what could have been an attacker's boat.

Divers and anti-terror experts continued to search for more evidence around the tanker. Shipping executives said the discovery of human remains would prove it was a suicide bombing.

The Limburg is now anchored in the Gulf of Aden off the coast of Mukalla, some 800 km (500 miles) southeast of Sanaa.

"All that remains to prove that it was a suicide bombing is to find human remains. But this is a shark-infested area and the ship had been moved several times since the incident," said a Western shipping executive who has access to the U.S. and French anti-terror experts in Mukalla.

France has said it expects Yemen to find and punish those responsible. Yemen has deployed naval ships and helicopters to protect foreign vessels in its waters.

A Western source told Reuters the oval-shaped hole in the hull was believed to be 11 metres (yards) high and eight metres wide with at least seven metres below the water line.

"This is more or less the same size of the hole on the Cole and the attackers have probably used the same tactics," he said.

Other shipping sources said the attackers must have had prior information about the Limburg as they struck the only full tank on its starboard.

The Limburg has a capacity for two million barrels. "I keep asking myself why the hell this particular tank," one shipping source said. "And I don't believe in coincidence."

Another shipping executive said: "The tankers are like sitting ducks. They are slow, heavy and difficult to manoeuvre."

Yemen is the ancestral home of Saudi-born bin Laden. Earlier this year, the Arab country staged the only military strike against al Qaeda outside Afghanistan, the main target of the United States' anti-terror war. (Additional reporting by Mohammad Sudam in Sanaa)