The U.S. government has unveiled new rules requiring all travelers to and from the Western Hemisphere to show passports to enter the United States.

The rules will apply to Americans as well as Canadians, Mexicans and citizens of Bermuda who are currently exempt from passport requirements.

The Homeland Security Department says some alternate documents will likely be accepted, such as the official border-crossing card used by some Mexicans.

U.S. officials say the regulations are meant to strengthen border security in the aftermath of the September 11, 2001 terrorist attacks.

Under a proposed timeline, the rules will be phased in gradually over the next 33 months.

A State Department official acknowledged the rules may cause delays for people and goods trying to cross the border.