U.S. President Barack Obama declared tonight that authorities had uncovered a "credible terrorist threat" following the discovery of US-bound packages containing explosives aboard cargo jets.

Obama said both had been addressed to Jewish organisations in the Chicago area.

The events "underscore the necessity of remaining vigilant against terrorism," the President said.

The packages both originated in Yemen, but Mr Obama did not 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 national elections in which discussion of terrorism has played almost no role.

Mr Obama stepped to the podium in the hours after officials disclosed that authorities in Dubai had 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. Mr Obama did not identify any institution that had been targeted.

Several other cargo planes at airports along the Eastern Seaboard of the US were also 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, US 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 US-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 were intended to do harm."

DHS Statement on Increased Security Precautions: "As a precaution, DHS has taken a number of steps to enhance security. Some of these security measures will be visible while others will not. The public may recognize specific enhancements including heightened cargo screening and additional security at airports. Passengers should continue to expect an unpredictable mix of security layers that include explosives trace detection, advanced imaging technology, canine teams and pat downs, among others. As always, we remind the public to remain vigilant and report suspicious activity to local law enforcement."

Official TSA Statement on Suspicious Items Onboard Cargo Flights: "The Transportation Security Administration (TSA) is aware of and monitoring reports of potentially suspicious items onboard cargo flights that landed safely at Newark Liberty and Philadelphia International airports. Out of an abundance of caution the planes were moved to a remote location where they are being met by law enforcement officials and swept.”