Officials said the intelligence reports followed the discovery earlier this month of an empty SA-7 launcher near a desert base used by U.S. air forces in Saudi Arabia. The launcher was found by Saudi security police near Prince Sultan Air Base, near Riyadh, the Saudi capital.
The Saudis could not determine whether the launcher had fired a missile, and they destroyed it before U.S. military or intelligence officials could examine it.
One official said that intelligence report was given credence by Abu Zubaydah, the al Qaeda organization's operations chief, who was captured in Pakistan in March and who has been providing information about the terrorist group.
A U.S. official also said the portable missiles, which can be carried in small crates, "are fairly light and not difficult to obtain on the gray market."
"It's conceivable that terrorists could get them," the official said. "It is one of a number of possible threats that we need to be mindful and concerned about."
Officials said another worry was an interview in an Arabic-language newspaper with a senior al Qaeda terrorist. Abd-al-Azim al-Muhajir, a senior commander, told a reporter for London's Al-Sharq al Awsat in Pakistan last week that the terrorist group is planning a major attack against the United States.
Al-Muhajir said U.S. military operations in Afghanistan have "changed the nature of the action in the field, media appearances and training centers." However, he insisted, al Qaeda is not "finished."
Asked about new attacks against the United States, al-Muhajir said: "We pray to God, the glorified and exalted, to help us in the coming stage, that is the 'guerrilla warfare,' and in dealing with the aircraft. Thanks be to God that we have taken big strides in this."
Gen. Peter Pace, vice chairman of the Joint Chiefs of Staff, told reporters at the Pentagon that U.S. military forces are on alert for attacks by portable missiles.
"We take very seriously the fact that our opponents do have surface-to-air missiles, shoulder-fired surface-to-air-missiles," Gen. Pace said. "And we take precautions on the ground and in the air any time we have our aircraft arriving or departing."
He said there were no reports of U.S. aircraft taking surface-to-air missile fire in Saudi Arabia after the discovery of the SA-7 launcher.
"That does not mean it was not fired; it simply means we do not know if that particular weapon was fired at that location or simply dropped off there," he said.