A United Airlines flight bound for Los Angeles returned safely to Sydney airport today after a note raising fears of a bomb threat was found on board, officials said.
"UA Flight 840, carrying 246 passengers, was about 90 minutes out of Sydney when an object which raised some security suspicions was found on board," the airline said in a statement.
"As a precaution, the captain immediately returned to Sydney, landing without incident at 5:50 p.m. (8.50 a.m. Irish time)," it said.
The emergency related to a "perceived bomb threat to do with a note found on the plane", said a spokesman for Australian Transport Minister John Anderson.
Local media initially gave confused reports of why the Boeing 747-400 had turned back, including saying a silent alarm had been triggered inside the cockpit, indicating a possible attempt to take over the plane.
A spokeswoman for Australia's largest airport said a television report that an attempt had been made to break into the cockpit was "not true at all". Other television reported a bomb scare on board.
Television footage showed several emergency vehicles lined up along one of the airport's runways while rush-hour traffic around the airport was at a standstill.
Other footage from a road traffic helicopter showed the jet taxiing slowly along one of Sydney airport's main runways after it landed safely.
The plane was taken to a remote corner of the airport, where it was surrounded by emergency vehicles.
Passengers would resume their flight to Los Angeles at 11:30 a.m. on Wednesday (2.30 a.m. Irish time tomorrow), United said.
Security at Melbourne airport, Australia's second largest, was reviewed last year after a man armed with wooden stakes tried to hijack and crash a Qantas airways domestic flight.