There are reports that a United Airlines pilot has turned back to Sydney 90 minutes after take-off after a security scare on board the flight.
Sydney Airport has denied claims that the United Airline flight carrying 246 people was forced to land after an attempt to break in to the cockpit.
United Airlines flight 840 was bound for Los Angeles but turned back to Sydney as a precautionary measure about 90 minutes after takeoff, a United Airlines spokesman said.
The airline gave no further details.
New South Wales ambulance and fire services are on standby and nearby roads have been closed off, according to Sky News.