WASHINGTON — Some airlines were telling passengers today that new government security regulations prohibit them from leaving their seats beginning an hour before landing

The regulations are a response to a suspected terrorism incident on Christmas Day.

Air Canada said in a statement that new rules imposed by the Transportation Security Administration limit on-board activities by passengers and crew in U.S. airspace. The airline said that during the final hour of flight passengers must remain seated. They won’t be allowed access to carryon baggage or to have any items on their laps.

Flight attendants on some domestic flights are informing passengers of similar rules. Passengers on a flight from New York to Tampa this morning were also told they must remain in their seats and couldn’t have items in their laps, including laptops and pillows.

The TSA issued a security directive for U.S.-bound flights from overseas, according to a transportation security official.

The official said passengers traveling internationally could see increased security screening at gates and when they check their bags, as well as additional measures on flights such as stowing carryons and personal items before the plane lands.

Homeland Security Secretary Janet Napolitano said today that passengers flying to the U.S. from overseas may notice extra security, but she said the measures “are designed to be unpredictable, so passengers should not expect to see the same thing everywhere.”

A transportation security official said passengers traveling internationally could see increased security screening at gates and when they check their bags, as well as additional measures on flights such as stowing carryons and personal items before the plane lands.

A Nigerian passenger on a Northwest Airlines flight from Amsterdam allegedly attempted to start a fire as the plane prepared to land in Detroit on Friday, according to authorities. The incident has sparked a major international terrorism investigation.

Air Canada said it was limiting passengers to one carryon bag in response to a request from the U.S. and Canadian governments.

The airline advised U.S.-bound passengers to restrict their carryon item to “the absolute minimum” or to not carry any bag on board at all.

“Carriage of any carryon item will result in lengthy security delays for the customer,” the airline said.

U.S.-bound flights on all airlines are experiencing significant delays, said Duncan Dee, Air Canada’s executive vice president and chief operating officer.

A spokeswoman with Infraero, a Brazilian government agency that oversees airport infrastructure, said that airlines had been asked by federal authorities to add another layer of security for international flights originating in the country after the attempted attack in the U.S.

The official said that passengers would face an extra screening that would take place just before they boarded planes. She would give no more details, citing security concerns.

David Castelveter, a spokesman for the Air Transport Association, said the domestic airline industry has been in close coordination with the security administration since Friday’s incident and there will be increased scrutiny of passengers. He declined to comment on whether new regulations have been put in place.