ISLAMABAD, Pakistan, March 2 — Four people, including a United States diplomat, were killed today when a powerful bomb triggered by a suicide bomber went off in the southern port city of Karachi, according to Pakistani officials. The attack came just two days before President Bush is to visit Pakistan, but Mr. Bush said in New Delhi today that he would continue with his trip nevertheless.

The Scene in Karachi The blast occurred at 9:05 a.m. local time near the United States consulate and a Marriott Hotel in an upscale area of Karachi, the country"s largest city and a commercial hub. Forty-five people were injured in the attack, government officials said.

Pakistani officials said a vehicle was intercepted by Rangers as it tried to ram into the diplomat"s vehicle near the consulate. The massive explosion rocked the neighborhood; the attacker"s vehicle was completely destroyed, and the vehicle of the diplomat was thrown into the air and then tumbled over.

"It was a suicide bomber and we have started the investigation," Rauf Siddiqui, the Home Minister of Sindh Province, said in a telephone interview tonight. Earlier, Pakistani officials had speculated that the bomb was perhaps a time-controlled device.

In a statement, the United States embassy in Islamabad identified the diplomat as David Foy, a facilities maintenance officer at the consulate.

"According to the U.S. Consulate in Karachi, all other American staff are safe and accounted for," the statement said. "Iftikhar Ahmed, a Pakistani national employed at the Consulate, was also killed in the bomb blast".

Mr. Bush"s visit on Saturday will be to Islamabad, about 1,000 miles north of Karachi.

Mr. Siddiqui said the remains of the suicide bomber were found and investigators were looking at various aspects of the blast.

Mr. Siddiqui also said that there was only explosion in the actual attack, and that a second explosion was caused by a compressed gas cylinder in another vehicle.

"Terrorists only target those areas that can make an impact," Mr. Siddiqui said, responding to criticism about the fact that the attacker managed to penetrate through a heavily guarded area of the city. "Bomb blasts even occur in the green zone of Baghdad," he said.

Pakistani officials said the timing and magnitude of the bombing suggested that it was not just the work of local militant elements.

Witnesses described thick smoke clouding the area after the blast as wreckage and debris was spread all over. More than 20 cars were damaged.

A police official in Karachi told reporters that the vehicle used in the attack was stolen two years ago from the North Nazimabad neighborhood in Karachi, according to state-run media.

President Pervez Musharraf condemned the attack and expressed his sympathy to the families of the dead; he also sent a condolence message to Mr. Bush.

"We express our profound condolences to the family of David Foy and all the innocent victims of this despicable act," Mr. Musharraf said. "Our thoughts and prayers are with them in this difficult moment."