Stringent new security regulations affecting most tourists have been introduced at US air and sea ports.
The fingerprints of 24 million visitors will be checked each year
Everyone entering the United States with a visa will now have fingerprints and photographs taken and scrutinised.
People on the visa waiver scheme - such as tourists from much of Europe, Japan and Australia - are not yet affected, but those on work visas are.
Homeland Security Secretary Tom Ridge said the US aimed to be "open to visitors but closed to terrorists".
All 115 US airports that handle international flights and 14 major seaports are covered by the programme, under which customs officials can instantly check an immigrant or
visitor's criminal background.
A similar programme is to be launched at 50 land border crossings by the end of next year.
There was a mixed reaction at Dulles Airport, the main international entry point for Washington, with some of those fingerprinted saying they understood why the new checks were wanted.
ENTERING THE US
Visitors will still face travel document checks and questions regarding their stay
Visa holders will now also have two fingerprints scanned and a digital photo taken
Decision will then be taken on whether to admit them to US
Eventually, fingerprint and photo checks will also take place on departure
But Brazilian Carlos Pimenta, his bags piled high on a cart, complained: "It's very embarrassing for us. I don't think it's appropriate to do this kind of stuff."
The BBC's Matt Frei saw the new procedure in action at Dulles and says the new checks are scheduled to add only about 15 seconds to the entry procedure.
But he adds that the extra measures are another sign of the US trying to find the right balance between freedom and security in a post-11 September world.
Called US-Visit, or US Visitor and Immigrant Status Indicator Technology, the new scheme will check an estimated 24 million foreign visitors who enter the US each year through airports or seaports on tourist, business and student visas.
Digital technology will be used to take fingerprints and photographs, which will be electronically checked against a national digital database for criminal backgrounds and any terrorist lists.
The Department of Homeland Security (DHS) says the data will be
stored securely and made available only to "authorised officials on a
These include customs and immigration officials, DHS transport security officers and consular officers of the state department.
But access may also be given to other federal, state, local and foreign officials.
"Careful safeguards... will ensure that the data is not used or accessed improperly", a DHS policy statement says.
Eventually, the process will be repeated when the foreigners leave the country as an extra security measure and to ensure they have complied with visa limitations.
Mr Ridge launched the programme formally at
Hartsfield-Jackson Atlanta International Airport in the southern state of Georgia.
He said the system had been tested at the airport for several weeks and had proved a "resounding success".
The technology was "easy for travellers to use but hard for terrorists to avoid", he added.
The measures replace the old special registration programme, which was said to have discriminated against Muslims and people of Middle Eastern origin.
But claims of discrimination are still being made by some of those countries whose nationals are affected.
Brazil has made formal complaints and started fingerprinting and photographing all US citizens arriving at its main international airports.
The US visa waiver programme allows citizens mainly from European nations to visit for up to 90 days without visas.