Citizens of the United States, Canada, Mexico and Bermuda now need to present a passport to enter the United States when arriving by air from any part of the Western Hemisphere.
The US Department of State says the changes in travel document requirements are the result of recommendations made by the 9/11 Commission.
Their purpose is to "to obtain [US] national security benefits as quickly as possible, and to expedite the processing of arriving passengers", the department says on its website.
Phase two of the changes is expected to see the same requirements introduced for arrivals by land and sea by the beginning of 2008.
For more information visit the Department's website here.