French Prime Minister Jean-Pierre Raffarin has asked national carrier Air France to cancel US-bound flights for two days because of a threat to security.
According to reports, six flights have been cancelled on Wednesday and Thursday. At least three were bound for Los Angeles.
In a statement, Raffarin's office said the move was based on information "gathered in the framework of Franco-American cooperation in the fight against terrorism".
The French Interior Ministry said the flights were cancelled at the request of the US embassy in Paris. The United States is on a heightened state of alert ahead of Christmas and the Washington Post reported foreign airlines were under particular scrutiny.
Over the weekend Washington raised its nationwide alert level from "elevated" to "high", while security was also stepped up across Europe for fear of Christmas and New Year attacks.
The French flights involved on Wednesday included the daily run from Paris's Charles de Gaulle airport to Los Angeles, Air France flight 68, as well as other less regular services.
Delta Airlines flights
In New York City, the US airline Delta Airlines terminal at LaGuardia Airport was closed for about 90 minutes on Wednesday due to a security breach, said a spokeswoman for the Transportation Security Administration.
Air France also carries passengers with the US airline Delta under a code-sharing agreement.
A woman passenger set off a metal detector in the terminal. The passenger continued into the terminal before she could receive a secondary screening with a hand-wand. Security was unable to locate her and evacuated the area.
Every single passenger from the terminal re-screened.
The terminal was reopened once the re-screening process was complete. The security breach caused delays of up to an hour for about 10 flights.
In Paris, airport officials said the move was taken as a security decision, taken by Air France and the public authorities.
Cancelled flights include the daily
run and less regular services
Law enforcement officials in Los Angeles - the second-biggest airport in the United States after New York - said the Federal Bureau of Investigation and the US Department of Homeland Security were probing a threat, but refused to divulge details.
"We are taking all reports of any threats very seriously as we are facing the most significant threat we have had since September 11, 2001 and are reacting accordingly," said a source.
A report in the Washington Post on Wednesday had foreshadowed the drama, saying passengers arriving at "at least one major US international airport on Air France and flights from Mexico will be subject to more security procedures beginning today, according to an aviation industry official and two other sources."
Jitters prompted authorities in France to put its fighter planes on alert while in Britain thousands of police were patrolling the streets of London and a suspected bomb plot in Spain was thwarted at the last minute.
Spanish Interior Minister Angel Acebes said police had arrested two suspected members of the Basque militant group ETA, who had planned to blow up a train arriving in Madrid.
French Defence Minister Michele Alliot-Marie said on Tuesday security forces had been on "orange" alert, the third-highest level, since the beginning of December, meaning more patrols of airports, train stations and shops as well as churches, synagogues and mosques.
"We have also done so in regards to protecting our air space, with our fighter planes on permanent alert," she said. "It is a sensitive time of year," she said.