"Parties bringing weapons as well as fighter jets to Vietnam's Hoang Sa Islands not only violates Vietnam's sovereignty but also complicates the situation on the East Sea," Foreign Ministry spokeswoman Le Thi Thu Hang said at a press meet Thursday.
The East Sea is internationally known as the South China Sea.
Hang was responding to an inquiry regarding Vietnam's reaction to China sending bombers to the Paracel Islands earlier this month.
A picture of an H-6J bomber being checked by technicians on a runway of the Woody Island, part of Vietnam's Paracel Islands, surfaced on China's social media platform Weibo on August 13.
Previously, a blog named Chinese Military Aviation said the H-6J, which was not weaponized, landed on the Woody Island in early August. Several military observers said the geographical features in the photo matched with features on the Woody Island as well.
The Hoang Sa (Paracel) and Truong Sa (Spratly) archipelagos are inseparable parts of Vietnamese territory, Hang reiterated. Vietnam has full legal basis and historical evidence to assert its sovereignty over both archipelagos in accordance with international law, she added.
"We call for parties to responsibly contribute to the maintenance of peace, stability and security in the East Sea," Hang said.
China seized the Paracel Islands from what was then South Vietnam by force in 1974, and has been illegally occupying it since.