American Airlines has grounded all flights in the US due to an unspecified technical issue.
The Federal Aviation Agency said: "American Airlines is reporting a technical issue and has requested a nationwide ground stop."
The airline has not said why it was stopping all flights but Christmas Eve travellers face the possibility of significant disruption.
"An estimated timeframe has not been provided, but they're trying to fix it in the shortest possible time," the airline said on social media.
In a preliminary statement, the Federal Aviation Agency said: "American Airlines is reporting a technical issue and has requested a nationwide ground stop. Please contact American Airlines for more information."
Passengers on social media reported having their flights stuck on the runway at various airports and being sent back to the gate.
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