A United Airlines flight was forced to return to Sydney International Airport as a precaution because of a problem on board, the airline said.

The flight, which took off about 3pm (AEST), was 90 minutes into the flight when the pilot made the decision to return to Sydney, the spokesman said.

He would not detail the nature of any problems onboard and refused to confirm whether there were any problems with the plane's landing gear or in the cockpit.

The flight landed at about 5.45pm (AEST).

NSW Fire Brigades and the NSW Ambulance Service had multiple vehicles on the scene, and police closed roads leading to the airport.

The Age