Vietnam accused China of conducting prospecting operations in Hanoi territorial waters on Friday, despite repeated demands for Beijing to withdraw its vessels from this disputed area in the South China Sea.
Since early July, the Chinese ship Haiyang Dizhi 8, accompanied by an escort, has been conducting research in Vietnam's special economic zone, to which Hanoi responded with warnings to Beijing and the dispatch of its coastguard.
Vietnamese Foreign Ministry spokeswoman Le Thi Hang said the Chinese fleet "continues to expand" its operations.
"There is no overlap in rights or disputes in the so-called Wan'an Tan [Name of that maritime zone] as China has no basis in international law to claim this area," the spokeswoman said in a statement.
"Once again, Vietnam demands that China immediately cease its serious violations, remove all its ships from Vietnam's maritime areas and give up repeating these violations," he added.
China claims almost the entire South China Sea, despite claims from neighboring countries. In recent years, it has built seven reefs on artificial islands capable of receiving military installations. The new islands are close to those occupied by Vietnam, the Philippines and Taiwan.
Malaysia and Brunei are other territories vying for jurisdiction over islands and reefs, rich in fisheries and potential fossil fuel deposits.