German airline Lufthansa says one of its planes en route from Frankfurt to Madrid was forced to make an unscheduled stop in Zurich because of an "undefinable smell from the kitchen" of the aircraft.

Airport spokeswoman Jasmin Bodmer says the Airbus 320 landed today at 8.25am local time and passengers were able to exit the aircraft normally.

Lufthansa spokesman Peter Schneckenleitner says the crew's decision to divert to the Swiss city was "purely a precautionary" measure and there was no risk to the 85 passengers or six staff at any time.

Schneckenleitner said three crew members were taken to hospital complaining of respiratory irritations.

He says passengers are being rebooked on to other flights to Madrid.

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