An American woman who was found to be in possession of a pistol and bullets on a flight from Atlanta to Narita airport near Tokyo last week was denied entry to Japan, airport sources said Friday.

The woman, believed to be in her 30s, told Delta Air Lines crew during the flight that she had mistakenly brought the handgun and ammunition with her onto the plane, despite passing through security inspection at the southern U.S. airport, the sources said.

The cabin crew took the handgun from the woman until the plane's arrival on the afternoon of Jan. 3, the sources said, adding that she apparently owned it for self-defense.

The woman returned to the United States the same day she arrived, they said.

The case follows an earlier one at Narita in which an American co-pilot was found to be in possession of live bullets during a security inspection before departing the Japanese airport on Dec. 14, the sources said.

The American Airlines pilot, who said the bullets were in his luggage by mistake, left the ammunition with airport staff and worked as scheduled, they said.

Describing both cases as "serious incidents," the transport ministry asked U.S. transportation security authorities on Jan. 4 to take necessary preventive measures.

The ministry made similar requests following a series of incidents in 2017 in which live bullets were brought onto planes from the United States arriving at Narita and Haneda airports.

As Japan steps up security for the 2020 Tokyo Olympics and Paralympics, airport officials have said it is also important to make sure that inspections of incoming passengers at overseas airports be strengthened.