Vietnam objected to China's recent reported laying of undersea cables in the disputed Paracel Islands, saying it was a violation of Vietnamese sovereignty.
Radio Free Asia (RFA) quoted the Vietnamese Foreign Ministry saying the country deployed a coastguard vessel into another contested island chain in the South China Sea, the Spratlys, in an apparent response to the presence of Chinese maritime militia around a Vietnamese outpost there.
In Hanoi, Foreign Ministry spokeswoman Le Thi Thu Hang was asked about a BenarNews report on Monday, also carried by Radio Free Asia, that a Chinese ship was laying or repairing undersea cables near Chinese outposts in the Paracels.
The reporting was based on commercial satellite imagery and vessel-tracking software, and was cited extensively by Vietnamese state media this week.
"Vietnam has sufficient historical evidence and legal grounds affirming its sovereignty over the Hoang Sa (Paracel) and Truong Sa (Spratly) archipelagoes in accordance with international law," Hang told reporters, according to the state-run Vietnam News Agency.
"Therefore, any activity relating to the two archipelagoes conducted without Vietnam's permission are violations of its sovereignty and of no value," she said.
US-based experts interviewed by RFA said the cable work suggested that China was installing an undersea surveillance system for its occupied features in the Paracels, further militarising the region.
Vietnam and China both claim the Paracel Islands, a series of rocks and reefs in the north of the South China Sea.
( With inputs from ANI )