France has suspicions about financial transfers made immediately before Swiss refiner Petroplus filed for insolvency this week, energy minister Eric Besson said.

"The judicial system has to shine a light on exactly what took place," Mr Besson told France Inter radio.

The public prosecutor in Nanterre, west of Paris, opened an investigation on Wednesday into whether Petroplus illegally withdrew funds from its French unit before it filed for insolvency.

Petroplus denies all allegations of fraudulent bankruptcy.

The prosecutor's office, which had access to book-keeping documents of the French subsidiary when it was put under judicial protection, is looking closely at the transfer of €100 million.

"We have suspicions about the conditions of this bankruptcy and notably about some of the financial transfers which took place the day before, and the day before that," Mr Besson said.

Petroplus filed for insolvency on Tuesday, putting more than 2,000 jobs across Europe at risk, after it defaulted on $1.75 billion of debt.

Europe's largest independent refiner by capacity has been teetering since its lenders restricted credit late last year, a victim of thin refining margins and high debt that was a result of its private equity-backed business model.

Petroplus last week put its Petit-Couronne refinery, which employs 550 staff, up for sale and the French government has been trying to find a buyer. "Our main concern is maintaining activity and jobs," Mr Besson said.

Swiss group Klesch may be interested in buying Petit-Couronne, France Inter radio said without citing its sources.

A dismal outlook for the European crude processing industry, amid falling fuel demand, has prompted several companies to put assets on the market. But even rock-bottom prices have failed to attract bidders, even for good quality facilities.

French oil major Total halted the sale of its Lindsey refinery in the UK at the start of 2012 after failing to find a buyer over the past two years.

Reuters