Washington ? The Homeland Security Department is warning law-enforcement officers al-Qaida may be plotting to fly cargo planes from another country into such crucial targets in the United States as nuclear plants, bridges or dams, an agency official said Friday night.

Separately, the U.S. Embassy in Saudi Arabia said it would close its diplomatic missions in that country today for an undetermined period because of credible information terrorists are about to carry out attacks.

The United States also warned that Taliban insurgents in Afghanistan may attempt to kidnap American journalists working in that country.

“The U.S. intelligence community remains concerned about al-Qaida’s interest in carrying out attacks on us overseas,” said Homeland Security spokesman Brian Roehrkasse.

A Homeland Security official said the information about the cargo planes, first reported Friday by NBC News, came from a single source overseas.

“It has not yet been corroborated,” the official said, speaking on condition of anonymity. “We’re in the process of trying to corroborate this information.”

“We also remain concerned about threats to the aviation industry and the use of cargo planes to carry out attacks on critical infrastructure,” the official said.

Both the Homeland Security Department and the FBI were posting an advisory Friday night alerting state and local authorities to the threat, Roehrkasse said. The advisory also was being directed to officials responsible for security at such infrastructure facilities as nuclear plants, bridges and dams, he said.

The information about possible attacks in the Middle East came from a separate source than the information about possible attacks using cargo aircraft, the official said.

Roehrkasse said the color-coded terror alert would remain at yellow, the middle level on the five-color scale indicating an elevated risk of terrorist attack.

Critics have said the Transportation Security Administration, the agency responsible for aviation security, hasn’t done enough to make cargo planes safe. Those criticisms intensified when a New York shipping clerk packed himself in a crate and flew undetected to Dallas.