New Delhi (CNN Business) A rogue trader in Singapore cost Japan's Mitsubishi at least $320 million when unauthorized bets on the oil market went wrong.

The employee made a series of unauthorized trades starting in January this year, manipulating risk management systems to make the transactions look legitimate, the company said. But the losses on those trades piled up after crude oil prices started falling sharply in July.

Mitsubishi said it discovered the rogue transactions in August, while the employee was absent from work. The Japanese company said that losses from the transactions could rise further.

"Although PDS has already closed the position in question and determined how much was lost on the underlying derivatives, we are now examining the total amount of losses," it said.

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