U.S. President Barack Obama declared Friday that authorities had uncovered a credible terrorist threat against the United States after the overseas discovery of U.S.-bound packages containing explosives aboard cargo jets.

Obama said both had been addressed to "two places of Jewish worship in Chicago."

The disclosures triggered a worldwide alert amid fears that Al-Qaida was attempting to carry out fresh terror attacks.

"The events underscore the necessity of remaining vigilant against terrorism," the president said. The packages both originated in Yemen, but Obama did not explicitly assign blame to Al-Qaida, which is active in the Arab nation and has long made clear its goal of attacking the United States.

The events unfolded four days before the U.S. midterm elections, in which the discussion of terrorism has been almost completely absent.

Obama stepped to the podium in the hours after officials disclosed that authorities in Dubai intercepted an explosive device bound for a Chicago-area Jewish institution. The second package was aboard a plane searched in England, and officials said it contained a printer toner cartridge with wires and powder.

That second package was aboard a plane in the East Midlands, north of London.

Obama did not identify any institution that had been targeted.

Several other cargo planes at airports along the Eastern seaboard of the United States also were searched, and officials said no explosives were found.

An Emirates Airlines passenger jet carrying cargo from Yemen was escorted from the Canadian border to New York City by two military fighter jets, U.S. officials said. They said it was a precautionary action.

An FBI spokesman in Chicago, Ross Rice, said both suspicious packages had been
sent from the same address in Yemen.

The president refrained from assigning blame to Yemen's al-Qaida branch, but
officials who spoke on condition of anonymity said they were increasingly certain that was the source. The same group was responsible for the attempted bombing of a U.S.-bound airliner last Christmas.

White House counterterrorism adviser John Brennan later told reporters that the explosives were in a form that was designed to try to carry out some type of attack, but he provided no further details.

"The forensic analysis is under way," he said, adding, "Clearly from the initial observation, the initial analysis that was done, the materials that were found in the device that was uncovered was intended to do harm."

Brennan said national security officials in the U.S. and abroad responded quickly and have defused the packages. He said a series of searches of suspicious packages in U.S. cities was just a precaution.