The US has introduced new security measures for those flying from Mexico or Canada.

The measures, which started on 19 July, include enhanced overall passenger screening, heightened screening of personal electronic devices, increased security protocols around aircraft and in passenger areas, and the use of advanced technology, expanded canine screening and the establishment of additional pre-clearance locations.

Any electronic device larger than a smartphone must be removed from protective casing and fully charged, ready for potential inspection.

US Homeland Security said the enhanced security was in response to the threat of terrorism.

"The United States and the global aviation community face an adaptive and agile enemy," says a statement on its website.

"Terrorist groups continue to target passenger aircraft, and we have seen a 'spider web' of threats to commercial aviation as terrorists pursue new attack methods."

Homeland Security is also introducing "heightened security standards" for airports and aircraft.

The new measures are being put in place at around 280 airports worldwide that offer flights to the US.

Airlines have warned passengers to allow extra time at the airport to get through security.

Air Canada has warned passengers to allow extra time (Getty)

Air Canada has updated the advice on its website: “Due to security measures implemented by the US Department of Homeland Security (DHS), you are encouraged to arrive at the airport at least two hours before your scheduled departure when flying to the US to allow for additional screening time.

“Heightened security measures have been introduced by DHS that require enhanced screening of personal electronic devices such as tablets and laptops. All personal electronic devices larger than a smartphone will need to be easily accessible and have all cases, cover and/or packaging removed.”