The 4,191-tonne South Korean tanker carrying 11 South Koreans and five Myanmar citizens partially sank about 30 kilometres (20 miles) off the western port of Incheon after discharging its cargo there.
The coastguard has rescued five crewmen and recovered the bodies of five more, including one South Korean, two Myanmar citizens and two unidentified victims. A search is still underway for the six missing.
“We believe that the fuel vapour that remained in the tank is the likeliest cause of the blast, given statements from surviving crewmen and the company that owns the ship,” a coastguard spokesman told AFP.
“Investigations are still underway to reach a final conclusion.”
The crewmen told investigators that the explosion occurred while they were cleaning up fuel residue in the tank, the spokesman said, adding forensic experts were examining the ship.
The vessel usually transported diesel but was carrying petrol on Sunday, he quoted officials of the owners as saying.
The authorities were checking fingerprints to identify two victims whose bodies were badly damaged by the blast, he said, adding they were also considering DNA tests.