A fire broke out early Tuesday in a major oil refinery in the southern city of Basra, the Iraqi oil ministry said. Four workers suffered burn injuries. The fire at the Shuaiba refinery started at 7 am (0400 GMT) "due to an explosion," said Assem Jihad, the Oil Ministry spokesman. "Fire fighters and technical teams have controlled the fire, which hit the gas unit," Jihad told The Associated Press. An investigation was under way and production is continuing. Basra, where about 80 percent of Iraq's oil reserves are located, is Iraq's second-largest city, 550 kilometers (340 miles) southeast of Baghdad.