ATLANTA (AP) - Americans flying to Mexico, Canada and the Caribbean made sure to bring their passports Monday because of a new rule going into effect Tuesday that requires them to show one to get back into the country.

Only about a quarter of U.S. citizens hold valid passports, and most Americans are accustomed to traveling to neighboring countries with just a driver's license or birth certificate, which have long been sufficient to get through airport customs on the trip home.

The new regulations requiring passports were adopted by Congress in 2004 to secure the borders against terrorists.

Travelers at Hartsfield-Jackson Atlanta International Airport and other airports said they had no complaints about the requirement.

"I'd rather be going through a security check, than possibly being blown out of the air because of lack of security measures," John Golden of Columbus, Ga., who was headed to Cancun, Mexico.

Starting Tuesday, Canadian, Mexican and Bermudan air travelers, as well as U.S. citizens flying home from those countries or the Caribbean, must display their passports to enter the United States.

The only valid substitutes for a passport will be a NEXUS Air card, used by some American and Canadian frequent fliers; identification as a U.S. Coast Guard merchant mariner; and the green card carried by legal permanent residents. Active members of the U.S. military are exempt.

For now, the rules affect only air travelers. Land and sea travelers will not have to show passports until at least January 2008. Air travelers who cannot produce a passport will be interviewed by customs agents, who will decide whether to let them into the country.

"We're not seeing a panic from travelers because we've been pretty diligent in telling them for over a year that they need a passport. It's written on any piece of paper we have going out," said AAA spokeswoman Teresa Hildebrand.

Internet travel sites such as Expedia.com have posted warnings "in bold with exclamation point," said company spokeswoman Erin Krause, adding that agents followed-up with e-mails to customers traveling to the affected destinations.

Canadian consulate officials in the U.S. reported fielding hundreds of calls a day, most from the approximately 100,000 Canadian "snowbirds" who spend the winter in Florida or Arizona and feared they might not be able to fly back without passports, said Lawrence Barker, president of the Canadian Snowbird Association. (They can, Barker said.)

The State Department issued a record 12.1 million passports in 2006 and expects to issue 16 million more this year to meet the increased demand.

Mexican consulates are seeing a demand for passports three times higher than usual in some offices. In San Francisco on Monday, the line of people applying for passports at the Mexican consulate stretched around the block.

Cruz Garcia, a Mexican citizen living in Hayward, had been in line since 5 a.m.

"It seems important for the American government to know who comes and goes," she said. She plans to visit her parents in Mexico this summer and wants to be ready. "I don't want any glitches."

On the Net:

State Department: http://travel.state.gov/travel/cbpmc/cbpmc-2223.ht