The US is considering forcing foreign planes crossing its airspace to provide lists of passengers as part of tougher guards against terrorist attacks.
Airlines could be forced to change routes if they do not comply
Every flight entering US airspace, regardless of destination, would have to give full passenger details to be checked against US terror watch-lists.
Such details must now only be given by planes actually landing at US airports.
The plan has angered airlines flying to Canada and Mexico, who say routes may have to be changed at great expense.
The proposals come after US counterterrorism officials indicated that al-Qaeda may be planning to use foreign-based airlines to attack targets in the US.
Four domestic flights were hijacked on 11 September 2001 and deliberately crashed in New York, Washington and Pennsylvania.
An estimated 500 foreign aircraft fly over US territory every day.
The proposal to require all airlines concerned to provide passenger lists was put forward by the US Transportation Security Administration.
The terms are being discussed with the countries concerned.
If the measures come into force and airlines fail to comply they could be forced to re-route their flights.
The proposal comes two weeks after US authorities forced a Dutch KLM plane bound for Mexico to return to Amsterdam, after they objected to the presence of two of its passengers.
The two men were Saudi nationals whose names appeared on a no-fly list compiled by the US Department of Homeland Security.
The pair had allegedly attended the same Arizona flight school as Hani Hanjour, one of the hijackers involved in the 11 September attacks.
Following a tip-off from the Mexican authorities, the US government refused to allow the plane into its airspace.
European, Canadian and Mexican airlines operate most of the foreign flights that cross US airspace daily.
One of the airlines potentially affected, Aeromexico, claims that the new rule would violate international aviation agreements.