Vietnam has demanded that China withdraw an oil exploration rig deployed in the Gulf of Tonkin, where the two countries are still to settle on a line of control.
Vietnam's Foreign Ministry spokesman Le Hai Binh said in a statement yesterday that it had lodged a protest with the Chinese Embassy in Hanoi.
"Vietnam resolutely opposes and demands that China abandon drilling plans and immediately withdraw the Hai Duong 981 oil rig from this area, and that it not take additional unilateral actions that further complicate the situation," the statement said.
Mr Binh also said that a lighthouse on one of seven artificial islands China had built in the South China Sea was "illegal".
In Beijing, Chinese Foreign Ministry spokesman Hong Lei told reporters at a regular press briefing: "The activity is taking place in a maritime area that is under China's undisputed control. This is regular exploration, we hope those involved can view this matter rationally."
The rig, which China calls Haiyang Shiyou 981 and Vietnam refers to as Hai Duong 981, belongs to China's state-owned China National Offshore Oil Corporation (CNOOC). Its deployment in the summer of 2014 off the coast of central Vietnam saw clashes between Vietnamese and Chinese vessels, triggering bloody anti-Chinese riots in Vietnam.
It was the second time this year that Vietnam has protested against the rig's deployment. In January, the rig was detected in an "overlapping area between the two continental shelves" of Vietnam, and China's Hainan island, which had not yet been delineated. China at the time responded similarly, saying it was in "totally indisputable waters under China's jurisdiction".
China claims almost the entire South China Sea as its territory, but other countries - most critically Vietnam and the Philippines - have overlapping claims. The South China Sea is called the East Sea in Vietnam.
The dispute, especially after the 2014 clashes which saw Chinese vessels use water cannon on much smaller Vietnamese boats, has triggered anti-China nationalism in Vietnam. Hanoi has also drawn closer to the United States, with President Barack Obama due to visit next month.
The activities of the oil rig come on top of increasing clashes between Vietnamese and Chinese fishing boats. Vietnamese fishermen have complained of being rammed by Chinese boats. Last week, Vietnam seized a Chinese vessel carrying 100,000 litres of diesel fuel allegedly meant to supply Chinese fishing vessels, Vietnam's An Ninh Thu Do newspaper reported. The newspaper also said that Vietnamese coast guard vessels had, in the last two weeks of March alone, driven 110 Chinese fishing boats out of Vietnamese territorial waters.
In a report on Monday, Thanh Nien News quoted Major Pham Dinh Thanh of the Hai Phong Border Guard as saying that for the last several years, Chinese fishing boats had been penetrating 10-15 nautical miles into Vietn