Passengers at some overseas airports that offer U.S.-bound flights will soon be required to power on their electronic devices in order to board their flights. The measure is intended to enhance aviation security at a time of increased threats.

The measure does not affect Canadian airports, which are not considered by the U.S. to be overseas.

The Transportation Security Administration says it is adding the requirement that passengers coming to the U.S. from some airports must turn on devices such as cellphones before boarding. It says devices that won't power up won't be allowed on planes and those travellers may have to undergo additional screening.

"As the travelling public knows, all electronic devices are screened by security officers," the TSA said in the Sunday release announcing the new steps.

The release did not indicate why uncharged devices will not be allowed on board and the TSA refused to elaborate when asked directly by CBC News. However, a number of media outlets such as The Guardian and NBC News have suggested that officials are likely concerned that such devices may contain explosives.

American intelligence officials have been concerned about new al-Qaeda efforts to produce a bomb that would go undetected through airport security. There is no indication that such a bomb has been created or that there's a specific threat to the U.S.

Important to 'crank up' security

Homeland Security Secretary Jeh Johnson recently ordered the TSA to call for extra security measures at some international airports with direct flights to the United States. TSA does not conduct screening abroad, but has the ability to set screening criteria and processes for flights flying to the U.S. from abroad, according to a Homeland Security Department official, who was not allowed to discuss the changes publicly and spoke on condition of anonymity.

During an interview aired Sunday on NBC's "Meet The Press," Johnson declined to speculate on whether new security procedures called for overseas will be required at domestic airports in the future.

"We continue to evaluate things," he said. "The screening we have right domestically from one domestic airport to another is pretty robust, as the American travelling public knows. In this instance we felt that it was important to crank it up some at the last point of departure airports and we'll continually evaluate the situation."

TSA will not disclose which airports will be conducting the additional screening. Industry data show that more than 250 foreign airports offer nonstop service to the U.S.

Undetectable explosives

Aviation remains an attractive target to global terrorists, who are consistently looking for ways to circumvent airport security measures, the DHS official said. Some details on specific enhancements and locations are sensitive because U.S. officials do not want to give information "to those who would do us harm," the official said.

American intelligence officials said earlier this week that they have picked up indications that bomb makers from Yemen-based al-Qaeda in the Arabian Peninsula have travelled to Syria to link up with the al-Qaeda affiliate there.

Al-Qaeda in the Arabian Peninsula long has been fixated on bringing down airplanes with hidden explosives. It was behind failed and thwarted plots involving suicide bombers with explosives designed to be hidden inside underwear and explosives secreted inside printer cartridges shipped on cargo planes.

Over the past year, Americans and others from the West have travelled to Syria to join the fight against the Syrian government. The fear is that a fighter with a U.S. or other Western passport, who therefore may be subject to less stringent security screening, could carry such a bomb onto an American plane.

With files from CBC News