ISLAMABAD, Pakistan, Sept. 21 -- Pakistani officials said Sunday 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 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," 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 Sunday that the two Americans killed in the blast were members of the U.S. defense forces assigned to the U.S. Embassy here. 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-Qaeda and Taliban fighters.

"All roads lead to South Waziristan and Tehrik-e-Taliban," 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.

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.

The truck had been packed with 1,300 pounds of military explosives, mortars and other weapons, Malik said. The bombing was timed to coincide with the fast-breaking meal, when guards were eating and likely to be distracted.

Malik said the attack was intended to destroy the hotel, a center of social and political life in the Pakistani capital and a frequent choice of foreign visitors. The ambassador from the Czech Republic was among the dead, officials said.

Yemen's official Saba news agency said 16 people died in the incident, including six Yemeni soldiers, four civilians and six attackers. One of the civilians was an Indian woman at the embassy on business.

There were no immediate reports of American casualties. The embassy is located in the center of Sana, Yemen's capital, but the building is set far back from an outer security wall.

There was no immediate claim of responsibility.

Yemen is the ancestral home of Osama bin Laden. Al Qaeda has maintained a steady presence here, especially as fighters from the militant group return from Iraq.

The first explosion, at 9:15 a.m., resounded for miles and sent a plume of black smoke over the city.

"It shook everything in my home," said Saddam Hussein, a Yemen man living about 200 yards from the embassy. "One big explosion, then smaller explosions, and gunfire."

The vehicle bomb exploded at the main gate, said embassy spokesman Ryan Gliha. There did not appear to be any major damage to the embassy building, Gliha said.

U.S. officials still were trying to determine the nature of several explosions that followed the vehicle bomb, Gliha said. Hussein and other nearby residents said the secondary blasts sounded much smaller, like grenades.

The intermittent explosions and heavy gunfire continued for about 10 minutes after the first blast, as scores of Yemeni forces rushed to the scene.

A half-hour after the blast, intermittent gunfire still crackled. Helicopters circled the embassy. Ambulances painted in green camouflage carried injured to the city's military hospital.

The Yemen embassy has been the target of numerous attacks since 2002. In the most recent assault, three mortar rounds hit a nearby school for girls in March, killing a security guard and injuring about 20 girls and others. Other attacks include one in 2006, when a man armed with an automatic weapon opened fire outside the embassy, saying he wanted to kill Americans. Security forces shot him and captured him, without other injuries.

Yemen was also the scene in 2000 of the deadly suicide attack against the USS Cole.