ST. JOHN'S, N.L. -- Officials say a helicopter flying from St. John's to an offshore rig was forced to turn around because of an oil leak.

The province's petroleum board says the aircraft was about 20 kilometres outside the city when the seepage was discovered on Tuesday.

The helicopter was en route to the West Hercules oil rig, located about 460 kilometres east of St. John's, when its crew noticed a low oil pressure indication.

Seven passengers and two crew members on board were returned to the city without incident.

A preliminary investigation by rig's operating company, Statoil, indicates the leak may have been caused by a wrong sized part being installed during maintenance.