Vietnam protested a Chinese decision to begin drilling for oil in disputed Southeast Asian waters, calling the move illegal Monday and demanding that Beijing pull back from the area.
Beijing's deployment of its first deep sea rig was the latest in a series of provocative actions aimed at asserting its sovereignty in the South China sea that have raised tensions with Vietnam, the Philippines and other claimants.
The United States shares many of the regional concerns about China's actions in the seas, which are potentially rich in gas and oil.
Last week, President Barack Obama signed a new defense pact with the Philippines aimed at reassuring allies in the region of American backing as they wrangle with Beijing's growing economic and military might.
The China Maritime Safety Administration posted a navigational warning on its website advising that the CNOOC 981 rig would be drilling in the South China Sea from May 4 to August 15, in an area close to the Paracel Islands, which are controlled by China but Vietnam claims as their own.
It said ships entering a 4.8-kilometre radius around the area are prohibited.
Vietnam's foreign ministry said the area where the rig was stationed lay within Vietnam's exclusive economic zone and continental shelf as defined by the 1982 UN Convention on the Law of the Sea.
''All foreign activities in Vietnam's seas without Vietnam's permission are illegal and invalid,'' the ministry said in a statement. ''Vietnam resolutely protests them.''
Vietnam's state-owned oil company, PetroVietnam, demanded that China National Offshore Oil Corporation ''immediately stop all the illegal activities and withdraw the rig from Vietnamese waters.''
Many analysts believe China is embarking on a strategy of gradually pressing its claims in the water by seeing what it can get away with, believing that its much smaller neighbours will be unable or unwilling to stop them.
Vietnam has accused Chinese ships of cutting cables to its exploration vessels and harassing fishermen, as has the Philippines.
Chinese assertiveness puts Vietnam's authoritarian government in difficult position domestically because anger at China, an ideological ally, runs deep in the country.
This is exploited by dissident movements, who accuse the government of being unwilling to speak out against Beijing.
Tran Cong Truc, the former head of a government committee overseeing the country's border issues, said the latest Chinese move was especially provocative.
''This act by China is much more dangerous than previous actions such as cutting the exploration cable or fishing bans,'' he said.