"We do not see it as a militarisation by going through what has traditionally been an worldwide water space", said Mr Mattis of the U.S. ship movements through the South China Sea.
"The placement of these weapon systems is tied directly to military use for the objective of intimidation and coercion", Mattis said, adding, "China's militarization of the Spratlys is also in direct contradiction to President Xi Jinping's 2015 public assurances in the White House Rose Garden that they would not do this". "So that's where the security threat comes from for them", said Mintaro Oba, a former US diplomat focused on Northeast Asia.
"What we see it as, is a reaffirmation of the rules-based order".
Woody Island, which China calls Yongxing, is also claimed by Vietnam and Taiwan.
He returned to a theme that he and other senior United States officials have hammered home since Trump took office-that America is here to stay in the Asia-Pacific region and that allies should stick with Washington instead of aligning with Beijing.
"If the USA will continue to pursue a constructive results-oriented relationship with China, cooperation, whenever possible, will be the name of the game, and competing vigorously where we must".
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He also reiterated that the Pentagon was committed to working with Taiwan to provide articles and services needed for its self-defence, comments that are likely to anger China.
U.S. Secretary of Defence Jim Mattis speaks at the IISS Shangri-la Dialogue in Singapore June 2, 2018. It says military activities on the islands have been for self-defense purposes. "This is our priority theater", Mattis said.
"China's policy in the South China Sea stands in stark contrast to the openness our strategy promises, it calls into question China's broader goals", Mattis said, according to Reuters.
Mattis' address in Singapore was the second time he had attended the summit since becoming Pentagon chief.
Last Sunday, Beijing protested what it called a "provocation" after U.S. warships came within 12 nautical miles (around 22km) of the disputed Paracel Islands, among a string of islets, reefs and shoals over which China has territorial disputes with its neighbours. Beijing has repeatedly warned the United States against sending warships to patrol the South China Sea. Mr. Trump announced Friday that the nuclear-weapons summit he had canceled with North Korea's Kim Jong Un is back on. China protested the maneuver. China has not sent high-level officials to the three-day meeting, in an apparent attempt to deflect attention from its campaign to expand its sovereignty across virtually the entire South China Sea.
The US wanted complete denuclearisation of the Korean peninsula, he added.
On the Trump administration's relations with US allies and neighbors, including the recent move to place tariffs on Canada, Mexico, and the European Union, Mattis acknowledged there had been "some unusual approaches".