China stressed its opposition on Wednesday to Vietnam's military deployments on islands that the country has illegally occupied in the South China Sea, following the reported deployment of rocket launchers by Vietnam on several of the Nansha Islands.

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.

On Wednesday, Elizabeth Trudeau, director of the office of press relations for the US State Department, said the US is aware of the reports that Vietnam has deployed close-range missile systems in several of the outposts in the Spratly Islands (Nansha Islands).

"We continue to call on all South China Sea claimants to avoid actions that raise tensions, take practical steps to build confidence and to intensify efforts to find peaceful, diplomatic solutions to disputes," she told the daily briefing.

Trudeau acknowledged that it means for Vietnam to halt actions but stressed that the US calls on all claimants to avoid actions that raise tensions.

US President Barack Obama announced a lifting of the ban on arms sales to Vietnam during his first trip there in May. The measure has been perceived by many in China and the region as part of US rebalance-to-Asia strategy that aims to rally countries there to counter the growing influence of China.

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.

mojingxi@chinadaily.com.cn