US energy giant Chevron is trying to control an oil spill in waters off Rio de Janeiro state after the Brazilian government demanded a \"rigorous investigation,\" local news reports said Monday. The spill, which could amount to between 400 and 600 barrels, began last week in the Frade field, located 370 kilometers (230 miles) northeast of the Rio de Janeiro state coast, the reports said. Chevron, which could not be reached for comment, was quoted as saying that 17 vessels were working around the clock to assist the containment operation, which involves the use of skimmers. Frade is located near the Roncador field, operated by Brazil\'s state-owned oil giant Petrobras. Brazilian President Dilma Rousseff has demanded a \"rigorous investigation into the causes of the accident.\"