President Barack Obama declared Friday that authorities had uncovered a "credible terrorist threat" against the United States following the overseas discovery of U.S.-bound packages containing explosives aboard cargo jets. Obama said both had been addressed to Jewish organizations in the Chicago area.

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 long has made clear its goal of attacking the United States.

The events unfolded four days before national elections in which discussion of terrorism has played almost no role.

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 East Midlands, north of London.

Obama did not identify any institution that had been targeted.