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United Airlines has just been accused of allowing a male passenger to masturbate next to a 17-year-old girl on board one of its flights last year.

Monica Amestoy -- now 18 -- has filed a lawsuit against UA, claiming she was flying back to L.A. from a high school debate tournament in New York last October when the incident occurred.

According to Amestoy, represented by Gloria Allred, a male passenger seated across the aisle from her exposed his penis to her during the flight ... and masturbated.

Amestoy says she complained to a flight attendant multiple times -- and the flight attendant did nothing to stop the man from continuing his behavior ... she wasn't moved, and the passenger wasn't addressed in any way.

Amestoy says the man briefly stopped masturbating while the flight attendant was nearby, but quickly resumed once the flight attendant left.

According to Allred, the man in question has since been arrested, prosecuted, and convicted in connection with the alleged crime. Allred says the man has also been ordered to register as a sex offender.