No serious problems were reported at Canadian airports as new rules came into effect Tuesday requiring everyone flying to or from the United States to have a passport.

Air Canada spokeswoman Angela Mah said the airline has been informing customers about the passport rule on its website since October.

"For us at Air Canada, it really has been business as usual," she said.

At Toronto's Pearson International Airport, for example, almost every passenger to the U.S. came with a passport.

The few without passports provided other identification and were allowed to board their flights.

American customs officials are being flexible for now. Authorities say that for the next couple of weeks, they won't necessarily turn people away if they show up without passports.

"What we're doing is giving an informed compliance approach by letting the person who approaches us without the proper documentation still be admitted to the United States if we can make sure we can ascertain their identity and citizenship," said Robert Jackson of U.S. Customs and Border Protection.

Around 96 per cent of Canadian airline passengers are already using passports to enter the U.S. As well, 94 per cent of Americans are using passports to fly to Canada.

The new regulations also require U.S. citizens to show a passport before flying back into the States from Canada.

These regulations came in the wake of the Sept. 11, 2001, attacks — a move by Washington to make their borders more secure.

But many experts believe the new rules will do next to nothing to keep a determined terrorist out of the U.S.

Wesley Wark, an international intelligence and security expert teaching at the University of Toronto, said terrorists travel on expertly forged passports.

"I don't think anyone would argue that a passport will provide fool-proof protection," he said.

Canadian security expert John Thompson says, at best, the compulsory passport rule might make it a little more difficult for a terrorist to acquire a false passport, but that's all.

"It has an inhibiting effect because they have to acquire documentation and a fake ID so it does slow them down. But it doesn't prevent it."