Major Japanese airlines on Tuesday began implementing tighter airport security measures on U.S.-bound passengers, checking for explosives planted in personal computers and other electronic devices.

Passengers are randomly selected before they board aircraft for the new screening procedures launched in response to a U.S. request and covering carry-on electronic devices bigger than smartphones, including PCs, tablet computers, e-book readers and cameras, airport officials said.

Normal-sized smartphones and cellphones are not subject to the tests, which are held behind closed doors.