|Searches were conducted after several aircraft were grounded in Philadelphia, Newark and New York City [Al-Jazeera]
Security forces in the United Kingdom and the United Arab Emirates have intercepted two small packages containing explosive material that were being shipped by air from Yemen to "places of Jewish worship" in the United States, Barack Obama, the US president, has said.
The packages were discovered on Friday at East Midlands airport, in Nottingham, around two hours north of London, and at a FedEx sorting facility in Dubai, a major Gulf business hub. Both contained computer printer equipment packed with powder and attached to wires.
Police in Dubai said the package they found bore the hallmarks of al-Qaeda, and Obama has pointed the finger for the parcel plot at the extremist group's Yemen offshoot.
Dubai police also said that the ink cartridge found at the sorting facility was packed with pentaerythritol tetranitrate, or PETN, confirming an what Jane Harman, a Democratic congresswoman from California who was briefed on the incident, had told the New York Times newspaper earlier.
PETN is the same substance that was packed into the underwear of Umar Farouk Abdulmutallab, the Nigerian man who attempted to ignite a bomb on board an airliner over the United States on December 25 last year.
The police said the explosive materials were wired to a mobile phone SIM card hidden inside the printer.
"Although we are still pursuing all the facts, we do know that the packages originated in Yemen," Obama said in a press conference on Friday. "We also know that al-Qaeda in the Arabian Peninsula, a terrorist group based in Yemen, continues to plan attacks against our homeland, our citizens, and our friends and allies."
The discoveries came after a tip from Saudi Arabia, the White House said, triggering a major security alert on three continents as officials scrambled to check other cargo bound for the United states from Yemen.
In Yemen itself, the discoveries have prompted authorities to tighten security and hunt for those behind the plot. Al Jazeera's Hashem Ahelbarra, reporting from Aden, said there is a large security presence on the streets and roadblocks have been set up.
Cargo not subject to stringent screening
One package, found in the United Kingdom, was on board a UPS cargo plan, while the other, in Dubai, was found in a FedEx sorting facility.
Al Jazeera's Dan Nolan, reporting from Dubai, said that authorities there would be pleased that the package had been found before it was put on a plane but also concerned given the volume of air traffic that passes through Dubai.
|The ink cartridge of a Hewlett Packard printer found in Dubai wired and stuffed with explosives [Dubai Police]
Multiple flights come into Dubai from Yemen every day, and the UAE-run Emirates airline operates five flights a day directly to the United States, Nolan said.
Both UPS and FedEx said they had halted all packages being sent from Yemen to the United States while the incident is investigated.
Bob Ayers, an independent security analyst, told Al Jazeera that cargo is subject to less stringent security screening than passenger luggage. The screening of cargo has been a point of debate in the United States; in 2007, Congress directed the Transportation Security Administration to screen all cargo carried on passenger flights beginning this year, the Times reported.
"Cargo is in big pallets, it's wrapped, its prepared for shipment," Ayers said. "You can't x-ray the large pallet in many cases, you don't tear it apart because its already been pre-packaged, so cargo has always been less rigorously inspected than baggage going into a passenger aircraft."
In September, a large fire broke out in the cargo hold of a UPS cargo plane shortly after it took off from the Dubai airport. The plane crashed, killing both crew members.
Investigators suspect the fire may have started in a large shipment of lithium batteries, but they will probably now check to see if any cargo from Yemen was on board, Nolan said.
The tip by Saudi Arabia and subsequent discoveries prompted an international security scramble. Canadian and US fighter jets were scrambled to escort a passenger flight from the United Arab Emirates to New York City, and a UPS truck carrying two items from Yemen was stopped and searched in New York as well.
Two UPS planes parked at airports in Philadelphia and Newark, New Jersey, were moved away from terminals and searched.
Britain is also on high alert.
"The UK authorities say they are urgently reviewing what security they need. They have already suspended direct flights between the UK and Yemen," Harry Smith, Al Jazeera's correspondent in the East Midlands, said.
In Yemen, authorities launched an investigation.
John Brennan, the US homeland security adviser, spoke with Ali Abdullah Saleh, the Yemeni president, on the phone and provided details about the intercepted packages, Al Jazeera's Ahelbarra said.
Saleh said Yemen would "do its best" to track down the source, Ahelbarra said.
The impoverished Arabian peninsula country has been battling Houthi Shia rebels in the north, a secessionist movement in the south and a growing al-Qaeda presence.
"Authorities here continue to reiterate that they are doing all they can to eliminate al-Qaeda from the country, amid growing international pressure," our correspondent said.