A CHINESE seismic survey ship that Vietnam accuses of disrupting British Petroleum's search for oil in the South China Sea has left the area following the appearance of two Vietnamese naval vessels in exploration Block 06, says British Petroleum. Located 430 kilometres off the southern tip of Vietnam, the block is administered by Vietnam, but lays within China's wide-ranging claim to much of the South China Sea. The potentially oil-and-gas-rich block is also adjacent to areas claimed by China near the contested Spratly Islands. Mr Michael Yeldham, chief executive of British Exploration Operating Co in Hanoi, said the Chinese ship, Fendou-4, began firing seismographic air guns into the sea late last week not far from where two companies working for BP were conducting a marine seismic surveys. Seismographic surveys are used to map underground geological features by creating shock waves that reflect against rock formations. Two or more vessels working in close proximity can disrupt the quality of data. Mr Yeldham said the Chinese ship left the area on Monday. ''The Vietnamese naval vessels appeared. They observed. They left. And soon afterward the Chinese vessel moved on,'' said Mr Yeldham. ''There was no engagement.'' He played down official Vietnamese suggestions that the Chinese vessel had harassed the seismic ships working for BP. ''We were not harassed,'' he said. ''It was an unexpected visitor. But we were not harassed.'' In a statement published yesterday in Vietnam, the government accused China of ''causing interference'' in the block and ''seriously violating'' Vietnam's sovereignty. ''The owner of the Fendou-4 and the ship itself must bear full responsibility for consequences arising from its illegal activities,'' the statement said. Vietnamese officials could not be reached for comment. Block 06 is the same area where BP had a gas well blowout in February. The blow out, which forced the evacuation of the drill ship Actinia, continues to release raw natural gas into the atmosphere, Mr Yeldham said. BP is planning to drill a relief well to reduce the flow. British Petroleum's BP's partners in Block 06 drilling partnership are Oil & Gas Commission-Videsh of India (ONGC) and Statoil, the Norwegian state oil company.