"China's militarization of artificial features in the South China Sea includes the deployment of anti-ship missiles, surface-to-air missiles, electronic jammers and more recently, the landing of bomber aircraft at Woody Island", he said.
Speaking just 10 days before President Donald Trump is due to meet North Korean leader Kim Jong Un, Mattis also said the United States military continues to support diplomats pushing for the "complete, verifiable and irreversible" denuclearisation of the Korean peninsula.
Speaking at the same conference in Singapore, Lt Gen He Lei said, "Any irresponsible comments from other countries can not be accepted".
Defense Secretary James Mattis is warning China there will be consequences if it continues its military buildup in the South China Sea.
USA officials are also reportedly pushing worldwide allies and partners to increase their naval deployments in the vital trade route as China builds up its military presence on the disputed Paracel and Spratly islands, even if they stop short of a direct challenge.
Pentagon officials have long complained that China has not been candid enough about its rapid military build-up and using South China Sea islands to gather intelligence.
China has dispatched a low-level military delegation to an annual security conference in Singapore that has dwelled heavily in the past on Chinese activities in the South China Sea.
"I believe there are much larger consequences in the future", he said.
Gen Mattis said the Trump administration wanted a constructive relationship with China but would compete vigorously if necessary.
China has repeatedly warned the US against sending warships to patrol the South China Sea.
Under President Donald Trump's new National Security Strategy unveiled previous year, USA policymakers are now frequently pledging to defend a "free and open Indo-Pacific", a term that is more inclusive than the more widely used "Asia Pacific" as it encompasses the Indian Ocean and India, a key US ally that is also being threatened by Beijing's aggression.
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There is a dual-lens rear camera setup with a 12MP primary and 5MP secondary sensor along with an 8MP front-facing camera. The Lenovo-owned company today announced that it has partnered with Twitter to livestream the launch event in India .
China's Defense Ministry on Thursday said it still hoped for a sound and healthy relationship with the USA military.
Mattis also struck at one of the key, longstanding disputes between the USA and China, telling the conference that America will continue to provide defense equipment and services to Taiwan and oppose any effort to alter the status quo.
Mattis responded that the question reflected a fundamental disconnect with the way global tribunals have spoken on the matter.
Without naming China, she suggested the warships will cross into "territorial waters" claimed by Beijing and envisioned a potential encounter with its military.
The Pentagon chief gave only a brief mention of the negotiations for the planned June 12 summit in Singapore between Trump and North Korean leader Kim Jong Un.
On Sunday, two U.S. warships sailed close to the Paracel Islands in a long-planned operation created to challenge China's claims.
The U.S. military presence in the region is not limited to the Korean Peninsula.
Mr Mattis said there was little doubt about Beijing's intentions.
Tensions in the South China Sea are only warming up but Australia's options beyond a "vigorous diplomatic assertion" are limited, former foreign minster Bob Carr believes.
Gen Mattis said the issue of removing U.S. troops from South Korea was "not on the table" and that "our objective remains the complete, verifiable, and irreversible denuclearisation of the Korean peninsula".