The United States said on Monday it was unclear what caused a blast that gutted a French-flagged tanker in the Gulf of Aden but one senior U.S. official told Reuters it appeared to be an accident. The Yemeni government has ruled out an assault similar to the 2000 bombing of the U.S. destroyer USS Cole in the port of Aden, saying a fire caused the blast. But the ship's owners said terrorists using a boat might have staged a bomb attack on the tanker.

A senior State Department official seemed to back up the Yemeni government's view, telling Reuters on condition of anonymity that the blast appeared to have taken place on board the vessel "and was therefore probably an accident."

However, the official U.S. stance was that it was too soon to say what caused the explosion. "I don't think I can announce any conclusions at this point," State Department spokesman Richard Boucher told a news briefing.

"We have no information as to the cause of that damaged ship," Defense Secretary Donald Rumsfeld told reporters.

Yemen is trying to shed its image as a haven for militants from Osama bin Laden's al Qaeda network and has strived to improve contacts on security issues with the United States.

U.S. naval investigators and other experts have been sent to the scene, and Yemen and France have launched an investigation, a senior State Department official told reporters separately.

Last month, the U.S. Navy warned shippers of possible attacks by al Qaeda on oil tankers in the Gulf and Red Sea, which see oil exports of about 15 million barrels per day -- or one-third of international oil trade.

Some security experts also said the explosion that ripped through the Limburg on Sunday off Yemen was more consistent with an attack than an accident, after crew were reported seeing a small craft approach the tanker as a tug and pilot ship guided the vessel to port.

One expert noted that a large initial force was needed to make crude oil combustible. The tanker was transporting Saudi heavy crude oil loaded at the Saudi port of Ras Tanura.

Reports of a boat approaching the tanker revived memories of the Cole attack, which was rammed by suicide bombers in a boat packed with explosives. The attack killed 17 U.S. sailors

PHOTO CAPTION

The French-flagged supertanker Limburg burns in the Gulf of Aden early October 7, 2002, as Yemeni and French experts prepare to investigate an explosion the owners fear was caused by a terror attack. The explosion that ripped through the tanker is more consistent with an attack than an accident, its owners and security experts said. (Reuters TV)