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."