China National Offshore Oil Corp. deployed drilling rig HD-981 about 120 miles off the coast of Vietnam in disputed waters in early May. Both sides make competing claims over the territory, though the Chinese government has been accused of taking provocative action in response.
Nguyen Quang Ngoc, vice chairman of the Vietnam Historical Science Association, said he has an atlas from the late 1800s that shows the territory in question is under Vietnamese authority.
"China has no sovereignty over the [region]," he said.
No known map, he added, shows Chinese territory extending that far south.
The Chinese government accused its Western adversaries, the United States in particular, of being on the wrong side of the debate with its criticism of Beijing's reaction to Vietnam's claims.
Vietnamese nationals rallied in the United States, Europe, and at home to protest China's drilling rig, telling Beijing to keep its "hands off Vietnam."