SINGAPORE, May 29 (Reuters) - The United States said on Friday that China had placed mobile artillery weapons systems on a reclaimed island in the disputed South China Sea, a development that Republican Sen. John McCain called "disturbing and escalatory."
Brent Colburn, a Pentagon spokesman traveling with Defense Secretary Ash Carter, said the United States was aware of the weapons.
McCain, chairman of the Senate's Armed Services Committee, said the move would escalate tensions but not lead to conflict.
"It is a disturbing development and escalatory development, one which heightens our need to make the Chinese understand that their actions are in violation of international law and their actions are going to be condemned by everyone in the world," he said at a news conference in Ho Chi Minh City.
"We are not going to have a conflict with China but we can take certain measures which will be a disincentive to China to continue these kinds of activities," he said.
In Beijing, China's foreign ministry spokeswoman Hua Chunying said she had no information on the weapons.
U.S. officials say Chinese dredging work has added some 2,000 acres to five outposts in the resource-rich Spratly islands in the South China Sea, including 1,500 acres this year.
It has released surveillance plane footage showing dredgers and other ships busily turning remote outcrops into islands with runways and harbors.
Carter called on Wednesday for an immediate halt to land reclamation in the South China Sea and was expected to touch on the issue of maritime security and freedom of navigation again on Saturday in a speech at the Shangri-La Dialog security conference in Singapore.
China says the islands are in sovereign Chinese territory.
Pentagon officials said efforts by China and other claimant countries to turn reefs into islands in the Spratlys undermines international law and raises questions about their future plans and intentions.
"It creates an air of uncertainty in a system that has been based on certainty and agreed-upon norms," said Colburn, the Pentagon spokesman. "So anything that steps outside of the bounds of international law we see as a concern because we don't know what the ... motivations are behind that. We think it should concern everyone in the region."
Asian military attaches and analysts said the placement of mobile artillery pieces appeared to be a symbol of intent, rather than any major development that could tilt any balance of power.
"It is interesting and a point to watch. But it should be remembered they've already got potentially a lot more firepower on the naval ships that they routinely move through the South China Sea," one military attache said.
China claims most of the South China Sea. The Philippines, Vietnam, Malaysia, Brunei and Taiwan also claim parts of the vital trade route. All claimants except Brunei have military fortifications in the Spratlys.
(Reporting by David Alexander in Singapore, Mai Nguyen in Hanoi, Sui-Lee Wee in Beijing and Greg Torode in Hong Kong; Writing by Raju Gopalakrishnan; Editing by Nick Macfie)
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■US calls for land reclamation 'halt' in South China Sea
The US has called for an "immediate and lasting halt" to land reclamation in disputed areas of the South China Sea.
US Defence Secretary Ashton Carter told the Shangri-La Dialogue in Singapore that China's actions in the area were "out of step" with international rules.
China claims almost the whole of the South China Sea, resulting in overlapping claims with its neighbours.
Chinese officials have described US remarks on the South China Sea as "groundless and not constructive".
'Peaceful resolution'
Other countries have accused China of illegally taking land to create artificial islands with facilities that could potentially be for military use.
At the conference on Saturday, which was attended by defence ministers from across the Asia-Pacific region, Mr Carter said he wanted the "peaceful resolution of all disputes".
"To that end, there should be an immediate and lasting halt to land reclamation by all claimants," he said.
He acknowledged that other claimants - such as Vietnam, the Philippines, Malaysia and Taiwan, had reclaimed pockets of land or built outposts in the area, but said "one country has gone much farther and much faster than any other".
"China has reclaimed over 2,000 acres, more than all other claimants combined and more than in the entire history of the region. And China did so in only the last 18 months," he said.
"It is unclear how much farther China will go. That is why this stretch of water has become the source of tension in the region and front-page news around the world."
'Provocative remarks'
The US defence secretary also said the US would maintain a substantial presence in the region adding: "The United States will fly, sail, and operate wherever international law allows."
However, Senior Colonel Zhao Xiaozhuo, a member of China's delegation at the conference, said that China's actions were "reasonable and justified", and "it is wrong to criticise China for affecting peace and stability through construction activities".
Mr Carter's comments follow reports from US defence officials that China had put two artillery vehicles on one of the artificial sands it is creating in the disputed Spratley Islands area of the South China Sea earlier this month.
Asked about the US claims, China's foreign ministry spokeswoman Hua Chunying said she was "not aware of the situation" described, and urged the US to "stop making any provocative remarks".
China has previously said its work in the area is legal and needed to safeguard its sovereignty.