The sources added that naphtha was imported this month specifically for the cracker, which had started recently though an exact time frame could not be confirmed.
Petronas did not immediately respond to a request for comment.
The cracker is part of the US$2.7bil joint-venture oil refinery and petrochemical project known as Rapid – or Refinery and Petrochemical Integrated Development – located in Pengerang, in Johor.
The unit cracks naphtha into a chain of hydrocarbons, the bulk of it into ethylene which is predominantly used to produce plastics.
Rapid also houses a 300,000 barrel-per-day refinery apart from various petrochemical lines which can produce a total of 7.7 million tonnes a year of petrochemical products.
But the complex was hit by a fire in April when trial runs were being conducted. The fire-hit residue desulphrizer is still under repair. But Rapid is expected to have middle distillates supplies around August to September, although these fuels may not meet standard specifications, one of the sources added.