AL-MUKALLA, Yemen - The explosion which crippled a French supertanker off Yemen was not an accident, the ship's captain insisted today as French experts were set to join investigations into the exact causes.

"The way the explosion happened it could not be due to a technical problem," Hubert Ardillon told reporters.

"A member of the crew told me he saw a small boat approach and I believe him," the Limburg's master said.

"At least the first explosion occurred on the outside of the ship. I am positive about that. I saw it.

"I totally rule out the theory of an accident."

The French embassy in Sanaa said the damage was caused by a small boat packed with explosives that rammed the tanker on Sunday, recalling the Oct 12 Islamist attack two years ago on the US destroyer Cole in Aden harbour which killed 17 men.

Explanation needed

But the French foreign ministry stated there was no proof so far of an attack off Al-Mukalla.

A US official said Monday that initial evidence suggested the explosion that crippled the Limburg was accidental, as Yemen has claimed, and not terrorism.

"I am not talking about an attack, but I want an explanation for what happened," Ardillon added, noting he was on the bridge of the 330-metre vessel when he heard "three or four booms" aft.

French experts were due to arrive in Yemen to join the investigation into the exact causes.

"The ship has no weaknesses, that's clear. It was well maintained," said Ardillon in a hotel in Al-Mukalla.

He had earlier told a French newspaper in his home town of Rouen: "I've been told not to say anything. I do not have the right to talk."

Yemeni Transport and Maritime Affairs Minister Saeed Yafhi, who heads a crisis cell dealing with the incident, said he had been notified of the names of the members of the French team who were due to arrive at Al-Mukalla, in the southeastern province of Hadramaut, coming from Djibouti.

Yafhi stressed that Sanaa was keen on conducting a thorough investigation to determine whether the blast was caused by technical factors or by "an act from outside" the tanker.

Onboard explosion

In Washington, a senior State Department official said investigators from the US Navy were also en route to the stricken ship to assist French and Yemeni authorities.

"The indications tend to support" a theory advanced by Yemen that the explosion had been an accident, the US official told AFP on condition of anonymity.

However, he stressed that no final conclusions about the blast had yet been made.

"The indications are at this point that it was an explosion inside the vessel," the official said. "From the way the vessel looks, things are blown out, the plates are blown out."

That account squared with one offered earlier Monday by an AFP correspondent at the side of the ship who reported that the damaged metal around the breach - big enough for a man to pass through - appeared to have been blown out rather than in.

State Department spokesman Richard Boucher said that the explosion had "occurred onboard" the tanker, but would not elaborate.

The fire which followed the blast was under control and would soon be put out, according to a statement Monday from the ship's owners Euronav.

The company also said that the Yemeni authorities had confirmed that a Bulgarian member of the ship's crew was missing. "The 24 others are safe," the statement said.

Of the 25 crew - eight Frenchmen and 17 Bulgarians - 12 were injured. AFP