"All activities related to Vietnam’s Hoang Sa (Paracel) and Truong Sa (Spratly) Islands without Vietnam’s permission violate Vietnam’s sovereignty and hold no value," the ministry's spokeswoman Le Thi Thu Hang said at a press meet Thursday.

She was responding to inquiries regarding China apparently laying undersea cables between artificial features on the Paracel Islands.

Chinese cable ship Tian Yi Hai Gong sailed from a shipyard in Shanghai in late May and reached the Paracel Islands on May 28, according to satellite images and vessel tracking software, the Benar News, an online news service, reported on June 8.

The ship was "doing something related to undersea cables," which could be laying cables between the Tree Island, the North Island and the Woody Island, part of Vietnam's Paracel Islands. The ship then sailed to other locations.

China laying undersea cables could be for military purposes and potentially strengthen its ability to detect submarines, the paper said, citing experts. It could signal another step by China to militarize the South China Sea, it added.

Vietnam calls the South China Sea the East Sea.

China also laid cables connecting Vietnam's Woody Island to China’s Hainan Island in 2016, according to Reuters. The following year, China laid cables at the Fiery Cross Reef, the Subi Reef and the Mischief Reef of Vietnam’s Spratly Islands.

Vietnam has full legal basis and historical evidence to assert sovereignty over the Paracel and Spratly Islands in accordance with international law, Hang reiterated.

"Countries need to act responsibly, avoid complicating the situation and contribute to maintaining peace and stability in the East Sea," she noted.

Hang also said Vietnam was "paying attention" to a diplomatic note by the U.S. sent to the United Nations (U.N.) on June 3, in which it rejected maritime claims by China as "inconsistent with international law as reflected in the 1982 Law of the Sea Convention."

"Vietnam pays attention to the fact that several U.N. member nations recently have distributed documents to express their stance on the East Sea," she said.

The distribution of documents to express their stance is a usual activity for U.N. member nations, Hang noted, adding: "The international community as well as the U.N. values the fact that member states have stances that bolster and abide by international law, including the UNCLOS."