ISLAMABAD, Pakistan -- Pakistani officials said yesterday that 21 foreigners, including two Americans stationed at the U.S. Embassy, were among the victims of a massive suicide truck bombing Saturday night that destroyed a luxury Marriott hotel in the capital.

Prime Minister Yousaf Raza Gillani said the bomber's intended target was Mr. Gillani's official residence a block from the hotel, where newly elected President Asif Ali Zardari and other officials were gathered to break their daily Ramadan fast when the bomb exploded about 8 p.m.

"The purpose was to destabilize democracy," Mr. Gillani said.

As rescue teams combed the still-smoldering five-story building, officials put the death toll at 53, with an unknown number of people still unaccounted for. At least 266 people were injured. Most of the victims were hotel workers.

A spokesman for the Pentagon in Washington said yesterday that the two Americans killed in the blast were members of the U.S. defense forces assigned to the U.S. Embassy in Islamabad. Their names were not immediately released.

Pakistani officials said a contingent of 30 U.S. Marines was believed to be staying in the 290-room hotel.

A senior government security adviser, Rehman Malik, pointed the finger at Islamist militant groups based in South Waziristan, a volatile tribal area near the Afghan border. These groups have vowed to retaliate against the government for stepped-up military raids and for a series of U.S. military incursions in pursuit of al-Qaida and Taliban fighters.

"All roads lead to South Waziristan and Tehrik-e-Taliban," Mr. Malik said, referring to a militant group headed by Baitullah Mehsud, who has repeatedly vowed to attack the government after a truce with his forces collapsed last year.

Mr. Malik showed journalists a dramatic video of the attack, in which a large dump truck rammed into a metal barrier near the hotel and caught fire.

The video showed guards scattering, trying to put out the blaze, and scattering again when the driver kept going, detonating the huge blast.

Marriott said in a statement Saturday that several hotel guards who had gone out to examine the truck were among the dead.