Authorities uncovered a "credible terrorist threat" against the United States following the overseas discovery of U.S.bound packages containing explosives aboard cargo jets. President Barack 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.

One of the packages was found on a United Parcel Service (UPS) cargo plane at East Midlands Airport, about 160 miles north of London. The other was discovered at a FedEx Corp facility in Dubai.

Obama said a top aide had spoken to Yemen's president, Ali Abdullah Saleh, and that Saleh had pledged full cooperation in the investigation.

UPS and FedEx, the world's largest cargo airlines, said they were halting shipments from Yemen.

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

Suspicious packages were first found on cargo flights that landed in Britain and the United Arab Emirates on Thursday night, White House Press Secretary Robert Gibbs confirmed today, Friday.

Gibbs said Obama was first briefed on the matter Thursday night and directed U.S. intelligence and Homeland Security agencies to ensure American safety and determine whether the suspicious packages were part of a terrorist plot.

John Brennan, the deputy national security advisor for homeland security and counterterrorism, said authorities had identified the two packages containing explosive materials, which were isolated and "made inert".

Discovery of these packages occurred some fifty-five days after the crash of a UPS plane in the United Emirates.

UPS Cargo plane crashes near Dubai Airport

A UPS cargo plane with two crew members on board crashed shortly after takeoff Friday outside Dubai, officials said. There was no immediate word on casualties.

UPS spokeswoman Kristen Petrella said the Boeing 747-400 went down at about 8 p.m. in Dubai (12 p.m. EST). Flight 6 was en route to the UPS hub in Cologne, Germany, she said. Petrella said the plane had two crew members but the company has not confirmed any casualties.