Pakistan's top leaders were to dine at the Marriott hotel devastated by a truck bombing over the weekend, but changed the venue at the last minute, a senior official said Monday. A hotel official, however, denied there were any plans for a government dinner.

The attack that killed 53 people — including the Czech ambassador and two U.S. Defense Department employees — and wounded hundreds came at a time of strains in the U.S.-Pakistani alliance over increasing unilateral raids by U.S. forces in Afghanistan aimed at militants across the border in Pakistan. Two intelligence officials said Pakistani troops and tribesmen opened fire on two U.S. helicopters late Sunday after they crossed from Afghanistan into the northwest tribal region, where Taliban and al-Qaida militants are operating. The U.S. denied the report.

Dubai-based TV channel Al-Arabiya reported it had received a tape from a little known group calling itself "Fedayeen al-Islam" — Arabic for "Islam commandos" — claiming responsibility for the hotel bombing Saturday and demanding an end to U.S.-Pakistan cooperation against Islamic militants.

Pakistan's government is under U.S. pressure to crack down on the militants entrenched in the rugged, lawless tribal areas along the Afghan-Pakistan border, who are blamed for attacks on U.S.-led coalition forces in neighboring Afghanistan. Al Qaeda's leaders, including Usama bin Laden, are believed to be hiding in the tribal border region.

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Afghan Defense Minister Abdul Rahim Wardak, visiting Washington, said his government was proposing the U.S., Afghanistan and Pakistan create a joint military force with the power to operate on both sides of the border to fight militants. He said the idea was discussed at a meeting more than a month ago that also included Pakistani officials.

Interior Ministry chief Rehman Malik did not specify why the prime minister and president decided to move the dinner from the Marriott to the premier's house but said that decision was kept secret.

"Perhaps the terrorists knew that the Marriott was the venue of the dinner for all the leadership where the president, prime minister, speaker and all entire leadership would be present," he told reporters. "At the eleventh hour, the president and prime minister decided that the venue would be the prime minister's house. It saved the entire leadership."

However, a spokesman for the hotel owner said it had no plans to host a dinner for government leaders.

"We didn't have any reservation of such a dinner that the government official is talking about," Jamil Khawar told The Associated Press.

Malik had said Sunday the Marriott was likely targeted because the attack would get tremendous attention. But Prime Minister Yousuf Raza Gilani said the bomber attacked the hotel only after security kept him from reaching Parliament or the premier's residence, both less than a mile away.

The blast prompted foreign diplomatic missions and aid groups in Pakistan to review their security status. In Washington, the State Department said Pakistan had declined U.S. assistance in the investigation.

In a further sign of the country's deteriorating security situation Monday, gunmen kidnapped Afghanistan's ambassador-designate Abdul Khaliq Farahi and killed his driver in the main northwestern city of Peshawar, said a spokesman for the mission in the city.

The U.S. Embassy in Islamabad warned its employees to limit their movement to travel to and from the embassy and to shopping for essential items only. American consulates in Lahore and Peshawar reminded their personnel to avoid large hotels in those cities. The embassy warned all Americans to stay away from crowds, keep a low profile, and avoid setting patterns by varying times and routes for all required travel.

In Washington, State Department spokesman Robert Wood said the attack showed the need for Pakistanis, Afghans, and the U.S. to redouble efforts against extremists in this region.

"This was a heinous act that was committed by terrorists who have no interest in anything other than maiming and killing innocent civilians. And we're going to step up our efforts and work with the Pakistanis to do what we can," he said.

Most of the victims were Pakistanis, a fact likely to increase pressure on the government to stem the rising violence in the Muslim nation that many blame on the country's partnership with the U.S. in the war on terror.

Officials and experts said the scale of the blast and its high-profile target were hallmarks of Al Qaeda and its Taliban allies.

Al-Arabiya television said the group that claimed responsibility for the attack demanded an end to Pakistani-American cooperation against the militants and a halt to U.S. military operations in Pakistani tribal regions.

Amir Mohammad, an aide to one prominent Pakistani Taliban leader, Baitullah Mehsud, said the militant group was not involved and shared the nation's grief. Mehshud was blamed by the last government for a suicide attack in December that killed Pakistan President Asif Ali Zardari's wife, the pro-U.S. politician Benazir Bhutto. He denies that charge too.

"We have our own targets and we execute our plans precisely with minimal loss of irrelevant or innocent people," Mehsud was quoted as saying by his spokesman. "We have nothing to do with the Marriott hotel attack."

The intelligence officials who described Monday's U.S. helicopter incursion into the border region said informants in the field told them it took place about one mile inside the disputed and poorly demarcated border in the Alwara Mandi area in North Waziristan. They spoke on condition of anonymity because they were not authorized to speak to the media.

The helicopters did not return fire and re-entered Afghan airspace without landing, the officials said.

In Washington, Pentagon officials denied it.

"There was no such incursion, there was no such event," said Col. Gary L. Keck, Defense Department spokesman.

Pakistan's army said it had no information on the reported incursion.

A week ago, U.S. helicopters reportedly landed near Angoor Ada, a border village in South Waziristan, but returned toward Afghanistan after troops fired warning shots.

The alleged incident will likely add to tensions between Islamabad and Washington and comes as Zardari heads to New York to attend the U.N. General Assembly.