The United States fearfully cranked up its air transport system today but imposed a stringent anti-terrorist regime that excluded most foreign airlines.
A total 421 of the country's 451 airports were opened after being certified under the new regulations, said Federal Aviation Administration (FAA) spokesman Fraser Jones
The New York City area's three main airports -- Newark, Kennedy and LaGuardia -resumed operations in the late morning, a day after being shut down due to an unspecified FBI operation, he said.
"New York airports have been reopened," Jones said.
But Ronald Reagan Washington National Airport suspended flights indefinitely because it was too close to "key installations" such as the White House and the Pentagon, the airport said in a statement.
Boston's Logan Airport -- the departure point for the two Boeing 767 passenger flights that pulverized New York's World Trade Center -- also remained closed until further notice.
The FAA sent security notices to 129 foreign air carriers. "They must add additional security measures before they are allowed to resume operations in the United States," Jones said.
Foreign airlines must then affirm to their FAA inspectors that they have met the new standards, which are being kept under wraps.
The decision temporarily barred foreign airlines, except those from Canada, from flying into the United States. Only US operators were being allowed to fly passengers in from overseas.
Television pictures showed thousands of people in static queues at Chicago's O'Hare airport as they faced new rigorous interviews and luggage checks.
New security rules include a total ban on knives, searches of all airplanes and airports, no baggage check-ins except at the ticket counter and a reservation of boarding areas for passengers only.
US airlines said they were only slowly gearing up operations.
United Airlines was running 20 percent of its scheduled flights today a spokeswoman said. Asked when the system would be back to normal, she replied: "We do not have any idea on that yet."
United had no flights scheduled for non US destinations, she said, and there was no timetable as yet for resuming foreign flights.
US Airways said it was picking up the pace, with five of its planes coming back from Europe.