Hanoi has demanded Beijing remove military equipment from contested islands in the South China Sea, saying reported missile installations are a "serious violation" of Vietnam's sovereignty.
The warning follows a report from US network CNBC last week that China had installed anti-ship and air-to-air defenses on the Spratly Islands, which are also claimed by Vietnam.
China did not confirm the report but last week affirmed its right to build defense facilities in the South China Sea, of which it claims the majority.
Hanoi called Beijing's latest moves a threat to peace and asserted its historical and legal rights to the Spratly Islands, which it calls the Truong Sa islands.
"Vietnam asserts that all militarization activities, including the arrangement of missiles on Truong Sa islands seriously violates Vietnam's sovereignty," Vietnam's foreign ministry spokeswoman Le Thi Thu Hang said.
"Vietnam requests China not to militarize [and] withdraw military equipment that were illegally deployed on structures under Vietnam's sovereignty."
Vietnam and China have long sparred over their competing claims in the South China Sea.
Washington warned last week that Beijing would face "consequences" over its militarization of the waterway.
But Beijing defended its "peaceful construction" as necessary to protect its own sovereignty and security.