By STEVEN LEE MYERS WASHINGTON, Oct. 12 — 

An American Navy destroyer was rocked by an enormous explosion while entering a port in Yemen early today in what officials said appeared to be a deliberate act of terrorism. At least five sailors died and three dozen were injured, some of them severely. Twelve other American sailors were reported missing, and officials said the death toll would very likely rise.

The explosion, which occurred just after noon (3 a.m. eastern time), camefrom one of several smaller ships helping the destroyer dock at a refueling pier in the Yemeni port at Aden. A senior Pentagon official said the smaller ship, with two crew members aboard, had evidently been loaded with explosives in what appeared to be a well-planned suicide attack.

The explosion tore a hole, 20 feet by 40 feet, in the side of the Cole, a guided-missile cruiser based in Norfolk, Va., the officials said. The explosion caused extensive flooding, causing the ship to list slightly. By midday today, crew members were still struggling to control the flooding and keep the ship afloat.

President Bill Clinton said that if the attackers' motives were to dash hopes of a peace in the Middle East "they will fail utterly." Speaking outside the White House this afternoon, the President said American military people and diplomats in the Middle East had been placed on higher alert.

"If, as it now appears, this was an act of terrorism, it was a despicable and cowardly act," Mr. Clinton said.

He went on to condemn the killing of at least two Israeli soldiers by a Palestinian mob and said that whatever progress has been made toward a lasting Mideast peace "was not made through violence." He called on the antagonists to rein in their emotions and return to the negotiating table.

Senior administration officials in Washington did not immediately attribute the explosion to an act of terrorism. Secretary of State Madeleine K. Albright urged against jumping to conclusions, but officials at the Pentagon said there was little doubt that the Cole was damaged by a deliberate attack.

"We're not saying it was an accident," Lt. Comm. Kate Mueller, a spokeswoman for the Navy, said. "There is nothing that would make a small boat explode with enough force to make a 20-by-40 foot hole."

Officials said no one had claimed responsibility for attacking the Cole. Several radical Islamic organizations are believed to operate in Yemen, but the officials warned against jumping to conclusions about motives.

"If it is a terrorist attack, then obviously we will take appropriate action," Dr. Albright said at the State Department this morning. "I just urge a little caution."

She said she had spoken with the president of Yemen, who had offered to help with the treatment of those injured and with the investigation. Attorney General Janet Reno announced that the Federal Bureau of Investigation had already dispatched a team of investigators to Yemen to lead an inquiry — a clear sign the administration was treating the explosion as a potential terrorist act.

The Cole, on its way to the Persian Gulf, had just arrived at Aden when the explosion occurred. While the visit was scheduled to be brief, Navy officials had notified the government of Yemen in advanced of the stop. The senior Pentagon official said the Cole was attacked in a highly sophisticated operation that had to have been planned in advance, with access to information on the Cole's arrival.

"It was clearly a pre-planned, premeditated event," the official said.

The Cole's crew was already on a heightened state of alert because of the threat of terrorism, but the ship that exploded evidently did not raise suspicions because it was part of a flotilla helping the Cole moor at a refueling pier in the port. "It has a legitimate purpose for being in the area," the senior official said of the ship.

By evening in Yemen, the Cole's crew had managed to keep the ship afloat, but it continued to take on water. The injured were being treated at a hospital in Aden, but were expected to be evacuated soon to American military hospitals in Germany. A medical team was flying from Bahrain, headquarters of the Navy's 5th Fleet in the Persian Gulf. The Navy also dispatched other ships from the region to Yemen to help with the salvage effort of the Cole.