Beijing , China ( CNN ) -- Chinese authorities ordered beefed-up safety measures at the nation 's ports after a pipeline explosion spewed crude oil into the Yellow Sea , state media reported Friday . Ports will be required to perform checks and remove safety hazards ahead of loading , unloading or transporting oil and other hazardous chemical products , the transport ministry said in a notice on its website , according to the Xinhua news agency . China 's major ports have been ordered to complete safety evaluations by the end of the year . A state investigation has found that improper desulfurizer injections caused the oil pipeline explosion in the northeastern port city of Dalian , Xinhua said . Two pipelines exploded July 16 at Dalian Xingang Port , causing China 's biggest oil spill in recent history . Workers have been scrambling to prevent the 430-square-kilometer oil slick from further spreading into the ocean . The blast occurred as workers were injecting desulfurizer into the pipeline after a 300,000-ton Libyan tanker had finished unloading its oil , a state investigation found , said Xinhua . The desulfurizer , used to remove impurities from crude , was strongly oxidizing , the investigation found . The incident is still under investigation . The accident dumped 1,500 tons of crude and caused an oil slick on the Yellow Sea that has stretched to about 430 square kilometers ( 165 square miles ) , officials have said . That is still a fraction of the 2,700 square miles of the BP spill in the Gulf of Mexico . About 800 boats equipped with oil absorbers and dispersants have been dispatched . And crews are dumping 30 tons of oil-eating bacteria daily . Ship traffic at Dalian , China 's second-largest port for crude oil imports , was limited to avoid interference with the cleanup efforts , Xinhua has reported . And fishing , a major industry in the area , has been banned until the end of the summer . Beijing was optimistic that the cleanup would be complete by Friday but analysts argue it may take months . CNN 's Emily Chang contributed to this report .
An injection of desulfurizer is blamed for the explosion . The desulfurizer is strongly oxidizing . Workers are scrambling to prevent the spread of oil in the Yellow Sea .