The passenger, Ms Lucie Bahetoukilae, was supposed to be flying from Newark, New Jersey to Paris in France, but ended up flying more than 4,800km to San Francisco instead.

By the time the airline re-routed her flight to France, Ms Bahetoukilae, whose nationality is unclear, had been traveling for more than 28 hours, ABC7 News reported.

The ordeal for Ms Bahetoukilae, who speaks only French, began when she boarded the plane at Newark Liberty International Airport.

Unbeknownst to her, United had made a last-minute gate change, causing her to board the wrong flight.

Ms Bahetoukilae was neither notified via e-mail nor told by staff at the gate, and found someone sitting in her seat, 22C, after boarding.

Her niece, Ms Diane Miantsoko, told ABC7 News that the flight attendant looked at her boarding pass - which had Newark to Charles de Gaulle on it - and assigned her to another seat instead of verifying if she was on the correct flight.

Ms Miantsoko, who was not seeking a refund, said this was a serious security lapse on United's part.

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"With everything going on this country people have to be more careful," she added.

"They didn't pay attention. My aunt could have been anyone. She could have been a terrorist and killed people on that flight and they didn't know, they didn't catch it."

When contacted, United reportedly apologized and said it had "mistakenly put Ms Bahetoukilae on the wrong flight".

United has since refunded her and sent her a voucher for another trip to Paris.

A United representative also said the airline was working with its team in Newark to ensure such incidents do not happen again.

The incident is the latest in a string of controversies to hit the embattled American airline.

On April 9, United's handling of passenger David Dao on an overbooked flight at Chicago's O'Hare International Airport sparked a global backlash.

And in late April, a giant rabbit transported from London to Chicago in the cargo hold of one its planes was found dead upon arrival.