ISLAMABAD, Pakistan — Taliban militants based near the Afghan border and their Al Qaeda allies are the most likely suspects behind a massive truck bombing at Islamabad’s Marriott Hotel, officials and experts said today. At least 53 died in the explosion, including two American Defense Department employees and the Czech ambassador.

The truck sat burning and disabled at the hotel gate for at least 3½ minutes as nervous guards tried to douse the flames before they, the truck, and much of the hotel forecourt vanished in a fearsome fireball last night, according to dramatic surveillance footage released today.

The attack on the American hotel chain during Ramadan, among the deadliest terrorist strikes in Pakistan, will test the resolve of its pro-Western civilian rulers to crack down on growing violent extremism which many here blame on the country’s role in the American-led war on terror.

While no group has claimed responsibility, the scale of the blast and its high-profile target were seen by many as the signature of media-savvy Al Qaeda.

The Interior Ministry chief, Rehman Malik, said “all roads lead to FATA” in major Pakistani suicide attacks — referring to Federally Administered Tribal Areas, where American officials worry that Osama bin Laden and the Al Qaeda No. 2, Ayman al-Zawahri, are hiding.

A former government security chief for Pakistan’s tribal areas, Mahmood Shah, said that while the attack had “all the signatures” of an Al Qaeda strike, homegrown Taliban militants probably had learned how to execute an attack of such magnitude.

Al Qaeda was providing “money, motivation, direction, and all sort of leadership and using the Taliban as gun fodder,” he suggested.

A Pakistani intelligence official, speaking on condition of anonymity because he is not authorized to speak on the record to media, said investigators were examining just that theory.

Prime Minister Yousuf Raza Gilani of Pakistan said the attack was an attempt to “destabilize democracy” in Pakistan, which this year emerged from nine years of military rule, and destroy its already fragile economy.

Mr. Gilani also claimed that the bomber attacked the hotel only after tight security prevented him from reaching Parliament or the prime minister’s office, where President Zardari and many dignitaries were gathered for dinner.

However, the owner of the hotel accused security forces of a serious lapse in allowing a dump truck to approach the hotel unchallenged and not tackling the driver more clinically.

“If I were there and had seen the suicide bomber, I would have killed him. Unfortunately, they didn’t,” Sadruddin Hashwani said.

The bomb went off close to 8 p.m. yesterday, when the restaurants inside would have been packed with Muslim diners breaking their daily fast during the holy month of Ramadan.

The explosion wrecked a favorite spot for foreigners as well as the Pakistani elite that has been targeted twice before by militant bombings. The building — one of the few places outside the diplomatic district where American diplomats were permitted to socialize — was still smoldering 24 hours after blast, which also wounded more than 260 people.

Anti-American feeling is running particularly high following a series of strikes by American forces based in Afghanistan on Islamic militants nested in Pakistan’s tribal belt.

An American Embassy spokesman, Lou Fintor, said there was no evidence that Americans were the target.

Still, he confirmed that two Defense Department employees were among the dead and that a third American — a State Department contractor — was missing.

Three American Embassy employees and an embassy contractor were injured, Mr. Fintor said.

IntelCenter, a group which monitors and analyzes extremist communications, said a senior Al Qaeda leader, Mustafa Abu al-Yazid, threatened attacks against Western interests in Pakistan in a video timed to the recent anniversary of the September 11, 2001, attacks in America.

Mr. Malik, the interior minister, declined a reported offer of American assistance in the investigation, saying Pakistani agencies could cope.

Rescue teams searched the blackened hotel room by room today, finding several more bodies, and Mr. Gilani said the death toll had risen to 53. A Danish diplomat was also listed as missing and rescue workers said they expected to find more human remains.

Officials confirmed that the Czech Ambassador, Ivo Zdarek, was also among the dead. Mr. Zdarek, 47, only moved to Islamabad in August after four years as ambassador to Vietnam.

Mr. Malik said one Vietnamese citizen was also killed. The wounded also included Britons, Germans, and several people from the Middle East.

Mr. Malik told a news conference that the bomb contained an estimated 1,300 pounds of military-grade explosives as well as artillery and mortar shells and left a crater 60 feet wide and 24 feet deep in front of the main building.

The government released footage from a hotel surveillance camera showing the heavy truck turning left into the gate at speed, ramming a metal barrier and jolting to a halt about 60 feet away from the hotel.

Guards nervously came forward to look, then scattered after an initial small explosion.

Several guards tried repeatedly to douse flames spreading through the cab of the truck as traffic continued to pass on the road behind. There was no sign of movement in the truck and the footage played didn’t show the final blast.

Officials said vehicles carrying construction materials are allowed to move after sunset, meaning the sight of a dump truck near the government quarters might not have aroused suspicion.

The bombing came just hours after Mr. Zardari made his first address to Parliament since becoming president, less than a mile away from the hotel.

It drew condemnations from around the world, including from Bush, whose administration has pressured Pakistan to do more to put more pressure on militants using Pakistani soil to support the increasingly deadly insurgency in Afghanistan.

A recent series of suspected American missile strikes and a rare American ground assault in Pakistan’s northwest have signaled Washington’s impatience with Pakistan’s efforts to clear out militants. But the cross-border operations have drawn protests from the Pakistani government, which warned they would fan militancy.

The Marriott blast could prompt diplomats and aid groups in Islamabad to re-evaluate whether nonessential staff and family members should stay. U.N. officials met today to discuss the security situation and, for now, made no decision to change their measures, a spokeswoman, Amena Kamaal, said.

Mr. Zardari, who today was headed to New York to lead a delegation to the United Nations and was expected to meet with Mr. Bush during the week, spoke out against the cross-border strikes in his speech to Parliament. He condemned the “cowardly attack” afterward in an address to the nation.

Pakistan’s powerful army chief, General Ashfaq Parvez Kayani, joined the condemnation today, calling the attack “heinous” and saying the army stands “with the nation in its resolve to defeat the forces of extremism and terrorism.”

The army has staged offensives against insurgents in the nation’s northwest that have drawn revenge attacks by Taliban militants.

The country’s deadliest suicide bombing was on October 18, 2007, and targeted ex-Prime Minister Benazir Bhutto — Mr. Zardari’s wife — who survived. It killed about 150 people in Karachi during celebrations welcoming her home from exile.

Bhutto was assassinated in a subsequent attack on December 27, 2007.

The last big attack in Islamabad was a suicide car bombing in June outside the Danish Embassy that killed six people in apparent revenge for the publication in Denmark of cartoons depicting the Prophet Muhammad. Al Qaeda took responsibility.