The United States warned on Sunday that travellers who show up at overseas airports with electronic devices must be able to turn them on if asked to do so.

“Powerless devices will not be permitted onboard the aircraft,” the Transportation Security Administration (TSA) warned.

The move appeared to be connected to a rash of concern about threats to U.S.-bound flights. Last week, Homeland Security Secretary Jeh Johnson had asked the TSA to bolster security on direct inbound flights.

The TSA said that U.S.-bound travellers “may also undergo additional screening”. Neither Mr. Johnson nor the TSA said which airports were of most concern, noting only that it would be overseas airports with direct flights to the U.S.

U.S. media last week cited government sources as saying the heightened security came amid concern that terrorist groups in Syria and Yemen were working to develop bombs that are more difficult to detect.

U.S. officials were sharing information about the threat with other countries and the aviation industry, Mr. Johnson said.

ABC News reported last week that international airport authorities and airlines would also be directed to examine passengers’ shoes more closely and use more explosives-detection machines.

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