The warning follows a report from US network CNBC last week that China had installed anti-ship and air-to-air defences on the Spratly Islands, which are also claimed by Vietnam.

China did not confirm the new military equipment but last week affirmed its right to build defence facilities in the South China Sea, of which it claims the majority.

Hanoi called Beijing's latest moves a threat to peace and asserted Vietnam's historical and legal rights to the Spratly Islands, which it calls the Truong Sa islands.

"Vietnam... asserts that all militarisation 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 in a statement late Tuesday.

"Vietnam requests China... not to militarise (and) withdraw military equipment that were illegally deployed on structures under Vietnam's sovereignty," the statement added.

Vietnam and China have long sparred over their competing claims in the South China Sea, which is believed to be rich in oil and gas deposits.

Washington warned last week that Beijing would face "consequences" over its militarisation of the disputed waterway.

But Beijing defended its "peaceful construction" in the Spratly archipelago as necessary to protect its own sovereignty and security.

Though the Philippines, Taiwan, Malaysia and Brunei also have claims in the sea, Vietnam has remained the most vocal opponent to Beijing's build up in the sea.

Tensions reached a fever pitch in 2014 when Beijing moved an oil rig into a part of the sea claimed by Hanoi, sparking violent protests in Vietnam.