The fire started shortly before 4 p.m. Wednesday near the Raritan Center industrial park in Edison Township.
Middlesex County Emergency Management spokesman Rob Sklans said firefighters had hoped to have it fully contained by late Wednesday night.
Sklans said about 125 firefighters from Edison and surrounding towns and the New Jersey Forest Fire Service were on the scene.
Sklans said firefighters had difficulty getting to the fire because it was burning in an area that is inaccessible to vehicles.
No structures were immediately threatened and no injuries were reported.