A U.S.-bound United Airlines flight returned safely to Tokyo early Thursday after a passenger brandished an object that he said was a bomb, officials said. Authorities described the incident as a prank and said there were no injuries.

United spokesman Sarah Dornacker in Chicago said it was "unequivocally not a hijack" but "a security-related matter" that forced the return of 296 passengers and a crew of 20 on board Flight 58. She said about two hours after departure, the captain advised Tokyo he was returning to Narita Airport.Dornacker said the San Francisco-bound plane was a Boeing 747-100, and that the landing occurred without incident. She said nobody was injured. It was not immediately known where authorities had taken the passenger alleged to have staged the prank.

Transport and Foreign Ministry officials said the passenger, reportedly in the economy section of the plane, had begun shouting and exhibiting the object after the flight was over the Pacific and hours east of Japan.

Security officers at Tokyo's international airport said, however, no hijack signal was ever set off by the plane's flight crew.

The flight left Tokyo at 7:45 p.m. (3:45 a.m. Wednesday, MST ) and landed at Narita at 12:47 a.m. Thursday (8:47 a.m. Wednesday, MST).