The U.S. and Britain, citing concerns about terrorist attacks, are not allowing passengers on some flights from mostly Middle Eastern and North African countries to bring laptops, tablets and certain other devices on board with them in their carry-on bags. All electronics bigger than a smartphone must be checked in.

Here are some questions and answers on the ban:

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Which Flights and Airports Does This Affect?

The U.S. ban applies to nonstop U.S.-bound flights from 10 international airports in Amman, Jordan; Kuwait City, Kuwait; Cairo; Istanbul; Jeddah and Riyadh, Saudi Arabia; Casablanca, Morocco; Doha, Qatar; and Dubai and Abu Dhabi in the United Arab Emirates. About 50 flights a day, all on foreign airlines, are affected.

The British rules apply to flights from Turkey, Lebanon, Jordan, Egypt, Tunisia and Saudi Arabia.

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Why Are These Countries and Airports Singled out?

U.S. officials say the decision was prompted by intelligence about potential threats to planes bound for the U.S. They are giving no details, such as whether a particular terror group prompted the ban.

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Why Does This Affect only Nonstop Flights, Not Those with Stopovers?

Flights with stopovers on the way to the U.S. are met with additional security checks and screenings, adding an additional layer of protection. Bennet Waters of the Chertoff Group, a security consulting firm, says the affected airports may have been singled out by the government because of the procedures and equipment they use to screen carry-on bags.

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Why Are Laptops and Tablets Considered more Dangerous Than Cellphones?

The U.S. Transportation Security Administration has not explained. But Waters says TSA and intelligence officials have probably concluded that cellphones are too small to be rigged with enough explosives to bring down an airliner.

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Have Laptops or Electronics ever Been Used to Attack Planes?

Bombs concealed in electronics have been an airline safety concern for decades. In 1988, Pan Am Flight 103 was blown up over Lockerbie, Scotland, by a bomb hidden inside a Toshiba cassette recorder and packed inside a checked suitcase in the cargo hold. All 259 people on board and 11 others on the ground were killed. More recently, a Daallo Airlines plane landed safely in Somalia last year after a bomb inside a laptop exploded in the passenger cabin and blew a hole in the fuselage. Only the suspected suicide bomber was killed.

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Couldn't A Laptop with A Bomb Inside Still Pose A Danger within The Cargo Hold?

Yes. A bomb inside the cargo hold could bring a plane down, airline security experts say. But they say the scanning technology used to screen checked luggage for bombs is generally more sophisticated than the X-ray machinery used to examine carry-on luggage.

Also, separating the bomb from the bomber could make it harder to detonate it in the air. A timer would be less reliable, because it could go off on the ground if there were a flight delay, says Jeffrey Price, an aviation security expert and professor at Metropolitan State University of Denver. And a bomb with a barometric pressure switch that would detonate at a certain altitude could be set off by rough handling at the airport, Price says.

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Should Travelers Be Worried about Their Electronics Getting Stolen from Their Checked Luggage?

Yes, thieves are known to target expensive electronic equipment. Cheap luggage locks can be broken and canvas baggage can be cut into, says Anthony Roman, president of Roman & Associates, a risk management firm. He recommends using hard-shell suitcases that come with strong locks.