Vietnam demanded China move a controversial oil rig on Thursday and abandon plans to start drilling in waters where jurisdiction is unclear, the latest sign of festering unease among the two communist neighbors.
The $1 billion rig, which was at the center of a fierce diplomatic stand-off between the countries in 2014, had moved into an area of the Gulf of Tonkin in the South China Sea about which Vietnam said the two countries were still "executing delineation discussions".
China calls the rig Haiyang Shiyou 981. Vietnam refers to it as Hai Duong 981.
"Vietnam resolutely opposes and demands China cancel its plan to drill and immediately remove the Hai Duong 981 oil rig out of this area," Foreign Ministry spokesman Le Hai Binh said in a statement on the government's news website.
China claims most of the resource-rich South China Sea amid rival claims by Brunei, Malaysia, the Philippines, Taiwan and Vietnam.