Two Paris-bound Air France passenger jets, flying over the United States and Canada, were diverted because of unspecified threats.
Flight 065 from Los Angeles and Flight 055 from Washington were "subject to anonymous threats received after their respective takeoff", the airline said in a statement.
One of the jets carried out an emergency landing in Salt Lake City after it took off from Los Angeles on Tuesday.
The other plane, Air France 055, diverted and landed safely in Halifax, Canada, with 298 passengers and crew on board.
The FBI and local police were on the scene in Salt Lake City, the airport said. The US Federal Aviation Administration said all passengers were safe.
The FBI has said it found no evidence "which would lend credibility" to the threats against Flight 065, while the Royal Canadian Mounted Police in Halifax said on its Twitter feed that police dogs were searching Flight 055 for the possibility of explosives.
Air France said authorities were also tracking the "source of the telephone call". It was unclear if the same person had called in both threats.
Security concerns heightened
Friday night's attacks in Paris - claimed by the Islamic State of Iraq and the Levant (ISIL) - have raised security concerns around the world.
Bomb fears prompted police in Hanover, Germany, to call off a football match between Germany and the Netherlands two hours before game time on Tuesday. German Chancellor Angela Merkel had been due to attend.
One of Friday's targets was outside a Paris stadium where France was playing Germany in a friendly football match.
Meanwhile, investigators on Wednesday were looking for a second assailant on the run from the Paris attacks, the French prosecutor's office said.
AFP reported a video reportedly shows the man with two brothers - Brahim and Salah Abdeslam - who have been linked to the deadly assault. The newspaper Le Monde reported witnesses said three people were in the car used to attack restaurants and bars in the east of the city on Friday.
The 26-year-old Salah is currently being sought internationally, while Brahim blew himself up during the attacks that killed 129 people.
Source: Al Jazeera and agencies