The Homeland Security and State departments yesterday suspended two programs that allow some foreigners to enter the United States without a visa, in an effort to thwart an alleged terrorist hijacking plot that could have exploited that loophole.

The action, which took effect yesterday at 11 a.m., means foreigners will have to obtain U.S. visas, a process that now requires a face-to-face interview, to board international flights that have connecting stops at U.S. airports.

One program called "International to International" allowed a foreigner to fly, for example, from Rio de Janeiro to London with a layover in Miami without obtaining a U.S. visa. Upon arrival in Miami, the airline would be responsible for ensuring that the passenger made a connecting flight departing the United States. A second program, called "Transit Without Visa," allowed foreigners to take a U.S. domestic flight before leaving the country.

The agencies said they plan to reinstate both programs in the future and will review them in 60 days with updated intelligence. If the programs are suspended for 60 days, more than 6,000 travelers could be affected, the Homeland Security Department said.

U.S. officials have long been aware of problems with the "transit" programs. Government watchdogs have criticized poor record-keeping by the airlines and the Immigration and Naturalization Service, who are supposed to ensure that foreigners do not abscond upon arrival in the United States, according to government reports. In one instance last year, an INS agent pleaded guilty to smuggling Filipinos into the country illegally at Los Angeles International Airport, with the help of several airline-contracted security guards who were supposed to be escorting the "transit" passengers to connecting flights.

Last week, the government delivered a memo to U.S. airlines warning of a hijacking plot, possibly involving al Qaeda, that suggested terrorists would try to use the "transit" flights this summer to enter the country without alerting authorities and hijack an airliner. The memo said terrorists working in teams of five might try to use weapons disguised as common items, such as cameras. Targeted places include the East Coast of the United States, Britain, Italy and Australia.

"Recent specific intelligence indicates that terrorist groups have been planning to exploit these transit programs to gain access to the U.S. or U.S. airspace without going through the consular screening process," the Homeland Security and State departments said in a joint statement yesterday. "The steps announced today are designed to augment security against possible terrorist threats and to protect U.S. citizens and foreign nationals who fly into and out of the United States."

The federal government would reinstate the programs "as soon as additional security measures can be implemented to safeguard the programs from terrorists," the agencies said yesterday.

American Airlines, Continental Airlines, Japan Airlines, Varig, Iberia, Delta Air Lines and United Airlines fly the most number of "transit" passengers. The U.S. airports most often used are in Los Angeles, Miami, New York, Dallas and Houston, according to government figures. Airports serving the Washington area do not connect "transit" passengers.

Several U.S. airlines reached yesterday, including American and Delta, said they were cooperating with the government's decision and declined to comment on the financial impact it would have. "We're studying it," Northwest Airlines spokesman Jeff Smith said. "We would have to do some analysis to determine the impact."

The agencies' decision will not affect "transit" passengers who bought airline tickets before July 24 with plans to fly before Aug. 5. Likewise, transit passengers with a return flight scheduled before Aug. 9 do not need a visa.

Citizens of Brazil, Mexico, Korea, the Philippines and Peru are most likely to be affected by the new restriction, the agencies said. The requirement does not apply to citizens of the 27 countries that already have agreements with the United States that allow their citizens to enter the country without a visa. Those include Britain, France and most other Western European countries.