WASHINGTON - US investigators who visited a blast-crippled Petronas chartered French supertanker off Yemen found fragments of a small boat on deck and residue that tested positive for TNT, a US defense official said today.

Crew members saw a small boat speeding toward the Limburg moments before an explosion rocked the supertanker on Oct 6, blowing a hole in its hull at the waterline.

US investigators "found pieces and parts of a small marine vessel on deck," the official said, speaking on condition of anonymity. "I believe that we had a couple of areas test positive for residue of TNT."

Prime Minister Dr Mahathir Mohamad had earlier said that according to information relayed to him by national oil company Petronas, the blast was caused by a fire as a result of a break in an electrical circuit.

"When carrying oil, this (the explosion) happens when there is a small fire. This is based on a report from Petronas. They do not suspect any sabotage," he had said.

The latest finding came after US investigators joined French and Yemeni experts in investigating the explosion.

The French Foreign Ministry said initial results of the investigation into the current incident "appear to lead to the conclusion that the Oct 6 blast on the French oil tanker 'Limburg' was due to an attack."

Terrorism attack

Senior State Department officials in Washington also said that investigators now believe that the explosion was an act of terrorism, reversing an initial assessment that it may have been the result of an accident.

"There are now lots of indications that it was caused by some bad people, terrorists," one official said, stressing, however that the probe had not yet been completed.

A second official echoed those comments, saying: "There are signs that it was caused by something other than an internal explosion, but there are no definitive results."

An Islamic Yemeni group claimed responsibility for the blast, which killed one crew member and injured 12 others.

The Aden-Abyan Islamic Army said in a statement received by AFP in the Gulf emirate of Dubai that the target was actually a US Navy frigate, but that the mistake made no difference "because the unbelievers' nation is one."

It said the Limburg was "going to supply the (US) Fifth Fleet for striking the brothers in Iraq."

The attack came amid renewed concerns that the al-Qaeda terrorist network is preparing a fresh round of attacks on the United States and its allies.

Probe started

Seven French, two US and an undisclosed number of Yemeni investigators have boarded the Limburg to begin their investigation.

The findings reported by the Americans would appear to back up initial reports by the French embassy in Sanaa that the explosion was caused by a small boat packed with explosives that rammed the tanker.

Yemen's Transport and Maritime Affairs Minister Said Yafhi on Thursday said that Sanaa was not ruling out any possibility as to the cause of the blast.

"We are leaving all options open," he told reporters. "We are not ruling out any option in this incident. We are not ruling out anything at the moment."

Yemeni authorities had insisted shortly after the blast that the explosion was an accident. AFP