The measure is part of the Departments' Western Hemisphere Travel Initiative (WHTI), itself the result of recommendations made by the 9/11 Commission, which Congress subsequently passed into law in the Intelligence Reform and Terrorism Prevention Act of 2004.

Under this first phase of the initiative's implementation, citizens of the United States, Canada and Mexico will also have to present a passport or other accepted document that establishes the bearer's identity and nationality, in order to enter or re-enter the United States by air from within the Western Hemisphere.

A separate proposed rule addressing land and sea travel will be published at a later date, and will call for specific requirements for travelers entering the United States through land and sea border crossings.

However, as early as January 1, 2008, US citizens traveling between the United States and Canada, Mexico, Central and South America, the Caribbean, and Bermuda by land or sea could be required to present a valid US passport or other documents as determined by the Department of Homeland Security.

While recent legislative changes permit a later deadline, the departments said that they are working to meet all requirements as soon as possible.

The State Department also said that it has dedicated additional resources and personnel to meet the increased demand for passports. In Fiscal Year 2006, the Department issued a record 12.1 million passports to American citizens, and anticipates issuing 16 million passports in Fiscal Year 2007.