WASHINGTON >> Travelers flying to the U.S. from nearly 300 international airports, including those in Mexico and Canada, are now subject to stepped-up security measures that include stricter screening for electronic devices larger than cellphones.

The regulations could include asking passengers to present larger electronic devices for inspection and prove that they can be powered on.

The Homeland Security Department demanded last month that airlines around the world step up security measures for international flights bound for the United States or face the possibility of a total electronics ban for planes. The deadline for some of those changes to take effect was today.

Airlines and aviation authorities responded by warning passengers to expect longer security screenings at airports.

“Enhanced screening measures are in effect,” read an alert on the Canadian Air Transport Security Authority’s website. It said that passengers flagged randomly for additional screening will be asked to remove electronic devices from protective cases for inspection, and possibly show they can be powered on.

Mexico’s aviation authority advised passengers on flights bound for the U.S. to arrive at the airport three hours early to comply with the new screening measures.

Toronto-based Porter Airlines, which operates numerous flights a day between the U.S. and Canada, informed frequent travelers of the new security measures in an email today.

“As of July 19, if you’re travelling to the U.S., the U.S. Department of Homeland Security requires you to take your personal electronic devices larger than a smartphone, such as your laptop and/or tablet, out of their protective cases and to turn it on, if asked,” they advised, adding that devices that failed to comply would not be allows onboard.

The new rules apply to roughly 180 foreign and U.S.-based airlines flying from 280 airports in 105 countries. The Department of Homeland Security says more than 2,000 international flights land in the United States each day.