Anonymous telephone threats against commercial airliners have caused a scare involving at least six international flights, including two flying from the UK to the US.

Authorities said the threats did not appear to be credible, and described searches done on the jets as a precaution.

In one instance, US military jets escorted an Air France flight into New York City after someone claimed a chemical weapon was aboard the aircraft, the FBI said.

"Out of an abundance of caution, Air France flight number 22 was escorted to John F Kennedy airport by US Air Force fighter jets following a phone threat," the FBI said in a statement.

"There were no incidents or hazards reported on board the flight by either the passengers or its crew. The plane has been cleared."

A Saudi Arabian Airlines flight arriving at JFK was also checked out because of another threat, authorities said.

A third threat was made against an American Airlines jet flying from Birmingham to JFK while it was still in the air, airline spokesman Kent Powell said. Authorities initially told the pilot to land and taxi to a remote area away from the terminal but later radioed that the threat was not credible and cleared the plane to go to the terminal, Mr Powell said.

At Newark Liberty International Airport, passengers were removed from a United Airlines flight after it arrived from Madrid, United spokeswoman Mary Clark said. The plane was inspected at a spot away from the terminal.

Atlanta-based Delta Air Lines said two of its international planes were threatened: a Paris-to-Boston flight and a London-to-Newark flight.