Travelers flying directly to the United States from overseas are being asked to turn on their cellphones and laptops before boarding their aircraft, officials said Sunday.

The security measure calls for passengers whose devices cannot be turned on — turning on proves a device is real — to be denied boarding.

The Transportation Security Administration announced the added layer of security, apparently in response to reports that terrorists are designing bombs hidden inside smartphones and laptops.

“As the traveling public knows, all electronic devices are screened by security officers,” the TSA said.

“During the security examination, officers may also ask that owners power up some devices, including cellphones. Powerless devices will not be permitted on board the aircraft.”

It wasn’t clear Sunday which airports would be included in this increased security effort.

Homeland Security Secretary Jeh Johnson said last week that he had ordered the TSA to implement tougher security measures, but didn’t give any specifics.

“I have directed [the] TSA to implement enhanced security measures in the coming days at certain overseas airports with direct flights to the United States,” Johnson said Wednesday.

“We will work to ensure these necessary steps pose as few disruptions to travelers as possible. We are sharing recent and relevant information with our foreign allies and are consulting the aviation industry.”