European airlines steered clear of Ukraine's airspace after a Malaysia Airlines plane crashed on Thursday in the restive east of the former Soviet state.
In France, a statement by junior transport minister Frederic Cuvillier said he told "French airlines to avoid Ukraine's air space as long as the reasons behind this catastrophe are not known."
Malaysian Airlines flight MH17 disappeared from radar contact around 1415 GMT as it was flying over eastern Ukraine with 295 people on board.
Ukrainian President Petro Poroshenko has not ruled out that the airliner might have been shot down, saying he considers the catastrophe a "terrorist act".
Leading airlines quickly announced plans to route planes away from the area.
Air France said it had "taken the decision to no longer fly over eastern Ukraine as soon as it heard of the event."
A spokeswoman for German flag carrier Lufthansa told AFP it has decided to immediately make a "wide detour" around the area, and added: "Our passenger's safety is our top priority."
She noted however: "There was not and there is not at present an order to avoid flights in Ukrainian airspace."
In London, a spokesman for the British Department for Transport told AFP: "Flights already airborne are being routed around the area by air traffic control in the region."
British Airways issued a statement saying: "Our flights are not using Ukrainian airspace, with the exception of our once a day service between Heathrow and Kiev," which is several hundred kilometres (miles) west of the disaster zone.
In April, the US Federal Aviation Administration (FAA) declared the troubled Crimean peninsula, which is south of the crash site, a virtual no-fly zone for US air carriers and pilots.
A similar no-fly order for European airlines was issued, also in April, by the European Organisation for the Safety of Air Navigation, or Eurocontrol.
On Thursday, an executive at the flight tracking website FlightAware said: "We're seeing many airlines ... transiting Ukrainian airspace, but all north or west of the FAA/Eurocontrol prohibited zone."
But according to a UK Civil Aviation Authority spokesman, "Eurocontrol has issued advice to airlines to plan routes that avoid the area" where the Malaysian jet crashed.