A new rule requiring U.S. airline passengers to show a passport upon their return from Mexico, Canada or the Caribbean took effect Tuesday, with few reports of stranded travelers.

Hartsfield-Jackson Atlanta International Airport, the world's busiest, reported no problems by midday. "I think we're looking at 100 percent compliance," said Customs and Border Protection spokeswoman Susan Shattuck.

Most travelers who forgot about the new requirement were allowed to enter after receiving a warning and a passport application. But their names were entered into the agency's computer system, and they will be scrutinized if it happens again.

"Nobody's being turned away," said Roxanne Hercules, an agency spokesman in Northern California. "These are mostly U.S. citizens coming home from traveling. We just are trying to inform them of the new rules."

Only about a quarter of U.S. citizens hold valid passports, and most Americans were 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.

At Miami International Airport, a 7-year-old boy and a 2-year-old boy traveling with family from the Caribbean did not have passports.

"He had a pacifier but no passport," Customs and Border Protection spokesman Zachary Mann said of the toddler. Both boys were let into the country after officers explained the new rules to their relatives.

At Chicago's O'Hare International Airport, some Canadians arriving from Toronto had no quibble with the new requirement. "It makes sense," Shawn Palmer said. "I guess it makes a little more secure for getting across."

Travel agents and airlines reported no major problems after warning travelers about the new rule for more than a year.

"So far, all is quiet on our front," said Kathy Gerhardt, spokeswoman for Carlson Wagonlit Travel.

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

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.