Defense Secretary Jim Mattis links hands with South Korea's Minister of Defense Han Minkoo, left, and Japan's Minister of Defense Tomomi Inada ahead of a trilateral meeting at the 2017 International Institute for Strategic Studies (IISS) Shangri-la Dialogue, an annual defense and security forum in Asia, on Saturday, June 3, 2017 in Singapore.
Mattis, arguably Trump's most important statesman as the new president hopes to slash the State Department, tried to allay the fears.
Lieutenant General He Lei, the head of China's delegation to the summit, said the "Asia-Pacific situation is generally safe and positive" but warned of "hotspot issues" flaring up from time to time.
While Hailey said the United States wants a negotiated solution to the North's nuclear program and isn't seeking regime change, she reiterated that "all options for responding to future provocations remain on the table".
Nevertheless, reversing or slowing North Korea's nuclear and missile programmes has become a security priority for Washington, given Pyongyang's vow to develop a nuclear-tipped missile capable of hitting the USA mainland.More news: US governors, mayors defend Paris accord after Trump's rejection
China's claims in the South China Sea, through which about $5 trillion in ship-borne trade passes each year, are contested by Brunei, Malaysia, the Philippines, Taiwan and Vietnam.
"The Trump administration is encouraged by China's renewed commitment to work with the worldwide community toward denuclearisation", Mattis said.
President Donald Trump - who frequently denounced China on the campaign trail - has turned to Beijing to help pressure Pyongyang, prompting broad concerns that America will go easy on China's maritime activities.
North Korea's missile tests, the recent spate of terrorist attacks and China's military build-up in the South China Sea are likely to dominate long-awaited talks between Australian and USA defence and foreign ministers this week.
China claims sovereignty over all of South China Sea.More news: Helped by key double-fault, Murray tops del Potro in Paris
"China will not be a bystander in the nuclear issue", she said.
Allies around the world have been concerned about the commitment of the United States since Trump took office on January 20 because of his "America First" rhetoric and expectations that he would concentrate on a domestic agenda. "We believe the United States can engage China diplomatically and economically to ensure our relationship is beneficial not only to the United States and China but also to the region and to the world", Mr Mattis said.
Overall, Mattis' speech struck a positive, hopeful tone for cooperation and peace in the Asia-Pacific region, where he and his predecessors have made it a priority to nurture and strengthen alliances and partnerships. "Countries must also do more to break up North Korean smuggling rings, and cut off the sources of funding North Korea uses to pay for the development of weapons of mass destruction and the means to deliver them", she said in a statement.
"We oppose countries militarising artificial islands and enforcing excessive maritime claims unsupported by global law", he said.
North Korea maintains that its ballistic and nuclear weapons programs are intended for deterrence, especially from USA military invasion. He said, U.S. can not and will not accept unilateral, coercive changes to the status quo.More news: Trump summons Muslim nations to confront 'Islamic terror of all kinds'
"The scope and effect of China's construction activities in the South China Sea differ from those of other countries in several key ways", Mattis said.
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