"We have barely imported diesel at all this year and in April and May we raised refinery use to between 83 and 85 percent," the source said speaking on condition of anonymity.

The boost in refinery usage preceded a nationwide trucker strike over rising diesel prices last month. It led Petrobras to lower diesel prices as the government slashed taxes on the fuel.

Meanwhile, private fuel importers, which have invested over the last year to increase their supply in the country, say they are struggling to compete with the fuel prices charged by Petrobras.

Petrobras price cuts have also fueled speculation that the company will struggle to sell 60 percent stakes in four refineries as planned. But Petrobras said five companies had signed non-disclosure agreements to potentially bid on the deal.