Pakistan's security officials put all of the country's airports on "red alert" Thursday after intelligence warnings of a suicide attack.
"We had very credible information of a suicide attempt in Islamabad which prompted this step," a senior security official told CBS News' Farhan Bokhari. The official spoke on condition of anonymity because he was not authorized to address the matter.
The security official said the heightened security was prompted by information gained through the interrogation of a recently arrested militant suspect, but he declined to identify the suspect or say when or where the person was taken into custody.
In a conflicting report, an airport official in Islamabad told the Associated Press that the threat came from an anonymous phone call. Col. Ashraf Faiz, a senior airport security officer, told the AP that the airport hit the panic button on Thursday after an unidentified man called an office of Pakistan's national airline, saying a suicide bomber was about to attack it.
At the country's largest airport, in the capital city of Islamabad, only passengers with valid tickets were allowed to enter the airport until Thursday afternoon. A government official said the emergency security measures were under constant review.
Deepening the sense of crisis, the U.S. government has barred its personnel from major hotels and even restaurants and urged all Americans in Pakistan to do likewise.
The United Nations and other foreign missions are mulling whether to crank up security precautions, and expatriate staff and dependents are bracing for instructions to quit the country.
British Airways, whose flights are usually busy with staff from the vast British consular section in Islamabad, has canceled flights to Pakistan indefinitely, citing security concerns.
Thursday's warning came less than a week after a in Islamabad which left at least 53 people dead and more than 260 injured.
A Western defense official in Islamabad told Bokhari there were reports of a number of al Qaeda plans "in the pipeline to attack locations across Pakistan to destabilize the country."
According to a report Wednesday in Pakistan's Daily Times newspaper, intelligence agencies informed the police that three explosives-laden trucks had entered the capital city - one of them known to have been destroyed in the Marriott Hotel attack last Saturday.
Citing anonymous sources, the newspaper said the whereabouts of the other two trucks remained a mystery, but they were believed to be within Islamabad city limits.
It was not immediately clear whether there was a link between the missing trucks reported by the Daily Times and the move by officials to put airports on "red alert" Thursday morning.
Speaking to CBS News on condition of anonymity, the Western official said al Qaeda's central goal was to force Pakistan's leaders to abandon their support for the U.S.-led war in Afghanistan.
The bulk of logistical supplies for U.S. and NATO forces in Afghanistan pass through Pakistan.
Despite the constant warnings of al Qaeda plots from Pakistani and Western officials in the region, it was another, far-lesser-known group which claimed responsibility for the Marriott Hotel attack over the weekend.
That group, which calls itself "Fedayeen al-Islam" (Soldiers of Islam) threatening further attacks on any entity deemed supportive of the United States.
In an English language telephone message to reporters, the group said "all those who will facilitate Americans and NATO crusaders … will keep on receiving the blows."
CBS News research shows the group has claimed responsibility for at least one previous attack in Pakistan, but that claim was later debunked by a more credible claim by al Qaeda.
Neither the veracity of the group's claim over the Marriott attack, nor its threat made Wednesday could be confirmed by CBS News. No further claims of responsibility had surfaced on the Internet or in other media Thursday.
The bombing and the new threats underscored the danger Islamist militants pose to Pakistan, where al Qaeda and Taliban fighters have established bases in tribal regions along the border with Afghanistan.
The U.S. has pushed Pakistan to crack down on the northwest bases, even launching its own attacks, but those American strikes have drawn sharp condemnation from Pakistani leaders, who say they kill civilians, fan extremism and violate their sovereignty.
Pakistan's army has threatened to resist future incursions, and the U.S.-led coalition in Afghanistan said that Pakistani troops fired at two American helicopters patrolling the border on Thursday.
A coalition spokesman said the choppers, which weren't hit, neither crossed the border nor returned fire.
The Pakistani military disputed that assertion, saying its troops fired warning shots when the two helicopters crossed over the border - and that the U.S. helicopters fired back. But President Asif Ali Zardari, speaking in New York, said his troops only fired flares.