The move, which shows a further stage of Hanoi's militarization of the Nansha Islands, will have a negative impact on regional peace and stability, observers said.
Intelligence shows that Hanoi has shipped the launchers to five bases in the Nansha Islands in recent months, Reuters reported on Wednesday.
The launchers have been hidden from aerial surveillance and have yet to be armed, but could be made operational with artillery rockets within two or three days, it said.
Foreign officials and military analysts told Reuters that they believe the launchers form part of Vietnam's state-of-the-art extra artillery rocket system, which was recently acquired from Israel.
"China has indisputable sovereignty over the Nansha Islands and their surrounding waters," the Foreign Ministry's Spokesperson's office said in a written reply on Wednesday.
"China has always firmly opposed the illegal occupation of parts of China's Nansha Islands and reefs by certain countries and their illegal construction and military deployments on these islands and reefs," it said.
Vietnam's Foreign Ministry said the information was "inaccurate" but did not elaborate.
Vietnam has illegally occupied 29 of about 50 islands and reefs in the South China Sea.
It has conducted construction and reclamation work on more than 20 of them since the 1980s, and the scale of the reclamation has increased in the past two years.
It also has built infrastructure, including runways and barracks, on the islands and reefs.
Jia Duqiang, a senior researcher on Southeast Asian studies at the Chinese Academy of Social Sciences, said it is Hanoi's latest effort to tighten its hold on islands in the South China Sea.
"By fortifying the islands with rocket launchers, Vietnam is keeping up its militarization of the region in a more aggressive way," he said.
Xu Liping, another Southeast Asian studies researcher with CASS, said Hanoi is trying to emphasize its determination to strengthen its illegal occupation of the islands.
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