The government suspended two programs Saturday that allow foreigners to transit U.S. airports without visas to catch connecting flights between international points.
The State Department said al Qaeda and other terrorism organizations had planned to use the programs as a way of getting access to flights to and from the United States.
Before the change, potential terrorists could have arrived in the United States without visas, thus eluding required checks against federal lists of terrorism suspects.
The action by the departments of State and Homeland Security was effective at 11 a.m. Saturday.
The departments "have received specific, credible intelligence ... that certain terrorist organizations including al Qaeda have identified the visa and passport exemptions of those programs as a means to gain access to aircraft en route to and from the United States," State Department spokeswoman Jo-Anne Prokopowicz said.
She said the intelligence included information from both the FBI and the CIA.
Homeland Security Secretary Tom Ridge said in a statement that "terrorists aim to exploit our vulnerabilities and freedoms."
He said the steps announced Saturday, "while aggressive, are an appropriate response to the threat. We know they will have an impact on international travelers, but we believe they are necessary in order to protect lives and property."
Airlines were instructed not to allow layovers by travelers under the transit-without-visa or the international-to-international transit programs. Homeland Security agencies also were acting to increase security at airports and on airplanes that normally carry and process passengers under the programs, the statement said.
The suspension does not affect passengers from 27 "visa-waiver" countries, mostly in Europe and the Far East.
Those most affected, who will no longer be granted visa-free entrance for airport layovers, are passengers from Brazil, Mexico, South Korea, the Philippines and Peru, the statement said.
Principal airports are the international airports in Los Angeles, Miami, New York, Dallas and Houston.