WASHINGTON (AFP) - Although, the president of Yemen, Ali Abdullah Saleh, was skeptical about whether an explosion in a U.S. destroyer in Yemen Thursday, killing at least six sailors and leaving 12 others missing, was a deliberate act, telling U.S. Secretary of State Madeline Albright by telephone that initial reports were that it "was not a deliberate act," U.S. officials are claiming that a small boat on a suicide mission caused the blast.

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The U.S. states that witnesses reported seeing two men standing at attention on a Zodiac-type inflatable rubber boat just before there was an explosion alongside the USS Cole at the port of Aden, according to reports cited in military channels, said a Pentagon official, who spoke on condition of anonymity.

"If, as it now appears, it was an act of terrorism, it was a despicable and cowardly act," President Bill Clinton said. "We will find out who was responsible, and hold them accountable."

"If their intention was to deter us from our mission of promoting peace and security in the Middle East, they will fail, utterly," he said.

Secretary of State Madeleine Albright said the United States would take "appropriate steps" to find those responsible if it was an attack.

"Rest assured that the United States will seek with all our resources to determine the facts surrounding this tragedy," Albright said, cautioning however that there had not yet been any claim of responsibility.

Clinton ordered U.S. navy ships in the region to put to sea and ground forces to step up security measures.

The Pentagon, meanwhile, detected the movement of a brigade-size force in Iraq, but a defense official said it did not appear to be in preparation for an attack.

U.S. Defense Secretary William Cohen said at least five sailors were killed, 12 others were missing, and 36 more were injured in the explosion, which occurred as the Cole was anchoring for a five to six-hour refueling stop.

"The guided missile destroyer Cole was apparently attacked by terrorists in a small boat," said Admiral Vernon Clark, the chief of naval operations.

"And I say 'apparently' because the investigation is going to have to reach the bottom-line on this situation. But from a personal point of view, and what I know about the ship and the events that have been described to me, I have no reason to think that this was anything but a senseless act of terrorism."

The rubber boat was among other Yemeni harbor craft that were assisting with the mooring of the Cole at a refueling facility in the middle of the harbor, he said.

Two men were observed on the boat that took mooring lines from the Cole to a buoy and then returned alongside the Cole, where it blew up, Clark said.

The blast tore through the Cole's half-inch-thick hull, ripping open a hole six meters (20 feet) high and 12 meters (40 feet) wide from the waterline almost to the deck of the four-year-old ship, one of the navy's most technologically advanced.

It flooded the engine room and an auxiliary compartment, and damaged a crew mess room and living area, officials said. The flooding was quickly contained and power was restored to the ship.

Three aircraft were dispatched to Yemen with medical teams, security personnel and people to assess the damage, Clark said.

A senior State Department official said it would take a day or two for forensics experts to confirm the cause of the explosion.

Cohen and Clark said they were aware of no intelligence warnings that an attack was in the offing.

"We are a global power. We have global responsibilities, and there are risks associated with that," Cohen said.

It was not immediately clear why the U.S. navy was making port calls in Yemen, a country with a record of violence and where the U.S. says Osama bin Laden has longstanding ties.

"I can just say this: We have been working to improve our relations with Yemen for some time. And I'm sure that that was at the heart of the motivation of the unified commander," Clark said.

U.S. ship visits to Yemen began in May, and there have been three others since then, Clark said.

Notification of the Cole's visit was relayed to Yemen as much as 10 to 12 days in advance, he said.

He said the Cole adhered to a detailed security plan and had armed guards on deck, but its commander had no reason to suspect that one of the craft involved in the mooring operation was a threat.

"We don't automatically suspect people that are sent forward to help us in an official way," he said.

Federal Bureau of Investigation (FBI) agents were on their way to Yemen Thursday to assist the CIA and the Defense Department in the investigation, said Attorney General Janet Reno.

The State Department said it was issuing a worldwide travel alert warning Americans following the blast.