The US government has warned of an increased risk of terror attacks on Americans at home and overseas.
It says al-Qaeda could try to hijack cargo jets and crash them into targets.
Terrorist bombings overseas and an increased volume of threats against US interests at home and in foreign countries led the Homeland Security Department and FBI to issue the public warning.
Of particular concern is "al-Qaeda's continued interest in aviation, including using cargo jets" to attack infrastructure such as bridges or dams "as well as targeting liquid natural gas, chemical and other hazardous materials facilities", the department said in a statement.
It said "increased vigilance here at home is prudent and may be key to deterring or disrupting terrorist attacks".
The State Department also issued a "worldwide caution" for Americans travelling abroad, urging that they "maintain a high level of vigilance" for possible terrorist attacks.
"We are seeing increasing indications that al-Qaeda is preparing to strike US interests abroad," the State Department said.
Neither threat warning included any specific times, locations or methods for a potential attack.
The Homeland Security statement, however, urges a heightened state of awareness during the final days of the Muslim holy month of Ramadan, which ends early next week, and during the US Thanksgiving travel season, which begins next week and ends in early January.