Vietnam has lodged a formal complaint to the UN over China's placement of a surface-to-air missile battery on a disputed island in the South China Sea.
The country's foreign ministry said on Friday it was deeply concerned by the Chinese deployment, which it said threatened regional stability.
"These are serious infringements of Vietnam's sovereignty over the Paracels, threatening peace and stability in the region as well as security, safety and freedom of navigation and flight," Le Hai Binh, Vietnam's foreign ministry spokesman, said in a statement.
The statement, sent to the Reuters news agency, said diplomatic notes had been issued to China's embassy in Hanoi and to Ban Ki-moon, the UN chief, to condemn China's activities.
Taiwan and US officials said on Wednesday the missile system had been deployed to Woody island, which is part of the China-controlled Paracel chain that Vietnam and Taiwan also claim sovereignty over.
Tsai Ing-Wen, Taiwan's recently elected president, said on Wednesday the move had created a "tense situation", but Wang Yi, China's foreign minister, said the reports were being exaggerated by Western media outlets.
At a summit of Southeast Asian leaders in California on Monday, Nguyen Tan Dung, Vietnam's prime minister, suggested to US President Barack Obama that the US take "more efficient actions" against militarisation and island-building in the South China Sea.
China claims nearly all of the South China Sea, including small islands that are hundreds of kilometres from its southern coast.
Source: Reuters And Al Jazeera