“Interested parties should work together to maintain peace and stability in the South China Sea region and refrain from taking any unilateral measures that would increase tensions,” Lo said on Wednesday.
A U.S. defense official also confirmed the “apparent deployment” of the missiles, first reported by Fox News.
Images from civilian satellite company ImageSat International show two batteries of eight surface-to-air missile launchers as well as a radar system, according to Fox.
News of the missile deployment came as Obama and leaders of the Association of Southeast Asian Nations concluded a summit in California, where they discussed the need to ease tensions in the region but did not include specific mention of China's assertive pursuit of its claims in the South China Sea.
“We discussed the need for tangible steps in the South China Sea to lower tensions including a halt to further reclamation, new construction and militarization of disputed areas,” Obama told a news conference.
China claims most of the South China Sea, which The South covers almost 1.4 million square miles. It contains an estimated 11 billion barrels of oil. About $5.3 trillion worth of trade and more than half the world’s merchant tonnage passes through it and it contains some of the world’s most important fisheries. Vietnam, Malaysia, Brunei, the Philippines and Taiwan have rival claims to the waters.
Beijing has been building runways and other infrastructure on artificial islands to bolster its claims. Washington worries that China has built up the islands with the aim of extending its military reach in the South China Sea. China says they will have mainly civilian uses as well as undefined defense purposes.
The United States has said it will continue conducting “freedom of navigation patrols” by ships and aircraft to assure unimpeded passage through the region.
Mira Rapp-Hooper, a South China Sea expert from of the Center for a New American Security, said it was not the first time that China has sent such weapons to the Paracels, under Chinese control since 1974.
“I do think surface to air missiles are a considerable development,” she said. “If they have been deployed they are probably China's effort to signal a response to freedom-of navigation operations, but I don't think it is a totally unprecedented deployment.”
A U.S. Navy destroyer sailed within 12 nautical miles of Triton Island in the Paracels chain last month in a move the Pentagon said was aimed at countering efforts by China, Vietnam and Taiwan to limit freedom of navigation. China condemned the U.S. action as provocative.
“Woody Island belongs to China,” said Ni Lexiong, a naval expert at the Shanghai University of Political Science and Law.
“Deploying surface-to-air missiles on our territory is completely within the scope of our sovereign rights. We have sovereignty there, so we can choose whether to militarize it.”
Taiwan President-elect Tsai Ing-wen said tensions were now higher in the region.
“We urge all parties to work on the situation based on principles of peaceful solution and self-control,” Tsai said.
The missiles arrived at Woody Island over the past week, Fox News said. According to the images, a beach on the island was empty on Feb. 3, but the missiles were visible by Feb. 14, it reported.
A U.S. official told Fox News the imagery viewed appears to show the HQ-9 air defense system, which has a range of 125 miles and would pose a threat to any airplanes, civilian or military, flying close by.
Asked about the report, Bill Urban, a Pentagon spokesman, said: “While I cannot comment on matters related to intelligence, we do watch these matters very closely.”
China's move is likely to rattle Vietnam the most because of its proximity to the Paracels and because of a history of maritime tensions with China that culminated in 2014 with a standoff after China moved a massive oil rig into disputed waters.
Al Jazeera with wire services