North Korea's continued ambition to produce an intercontinental nuclear missile makes the state "a clear and present danger," US Defence Secretary Jim Mattis has warned a meeting of world leaders in Singapore.

Mattis' remarks on Saturday came a day after Malcolm Turnbull said China should do more to curb the "reckless and dangerous conduct of North Korea" at the same conference.

On Friday, China blocked tough new sanctions against North Korea that the US pushed in the UN Security Council.

However, the Security Council did vote unanimously to add 15 individuals and four entities linked to the North's nuclear and missile programs to a sanctions blacklist.

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World leaders have been gathering at the Shangri-La Dialogue, sponsored by the International Institute for Strategic Studies, held in Singapore.

Talking at the summit on Saturday morning, Defence Secretary Mattis was unrelentingly critical of North Korea.

"North Korea's continued pursuit of nuclear weapons and the means to deliver them is not new, but the regime has increased the pace and scope of its efforts," he said, alluding to the North's series of nuclear device tests in recent years and an accelerated pace of missile tests seemingly aimed at building a missile with enough range to hit the US and Australia.

"While the North Korean regime has a long record of murder of diplomats, of kidnapping, killing of sailors, and criminal activity, its nuclear weapons program is a threat to all," Mattis said, adding, "As a matter of national security, the United States regards the threat from North Korea as a clear and present danger."

Mattis noted that last week the Pentagon conducted what it called a successful test of its missile defence system, which is being developed mainly with North Korea in mind.
An interceptor launched from coastal California soared over the Pacific on Tuesday, scoring what officials called a direct hit on a target missile fired from a Pacific test range. It was the first time the system had been tested against a missile of intercontinental range.

On Friday, Prime Minister Malcolm Turnbull used his speech at the conference to encourage China to build global trust by helping to halt the "reckless and dangerous conduct of North Korea".

Pointing to "now palpable tensions" on the Korean Peninsula and in the South China Sea - a reference to China's controversial maritime claims - Turnbull said while countries like Australia and Singapore might be "smaller fish", they value their sovereignty.

"Maintaining the rule of law in our region, respecting the sovereignty of nations large and small is the key to continued peace and stability," Turnbull said.

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"The rapid rise of a new power creates anxiety and China's rise had become the "topic of most intense debate."

China would best succeed, he said, by building on trust and co-operation.

U.S. Defence Secretary Jim Mattis addressed the Shangri-la Dialogue, an annual defence and security forum, in Singapore on Saturday. Photo/AP
U.S. Defence Secretary Jim Mattis addressed the Shangri-la Dialogue, an annual defence and security forum, in Singapore on Saturday. Photo/AP

"A coercive China would find its neighbours resenting demands they cede their autonomy and strategic space, and look to counterweight Beijing's power by bolstering alliances and partnerships, between themselves and especially with the United States.

"If we are to maintain the dynamism of the region then we must preserve the rules-based structure that has enabled it thus far," he said.

"This means co-operation not unilateral actions to seize or create territory or militarise disputed areas."

While courting China's help in reigning in Pyongyang, Mattis also admonished Beijing for its actions in the South China Sea.

He criticised what he called China's disregard for international law by its "indisputable militarisation" of artificial islands in the disputed seas.

"We cannot and will not accept unilateral, coercive changes to the status quo" in the South China Sea, he said.

Overall, Mattis' speech struck a positive, hopeful tone for co-operation and peace in the Asia-Pacific region, where he and his predecessors have made it a priority to nurture and strengthen alliances and partnerships.

"While competition between the U.S. and China, the world's two largest economies, is bound to occur, conflict is not inevitable," he said.

"Our two countries can and do co-operate for mutual benefit. We will work closely with China where we share common cause."

Mattis made no mention of President Donald Trump's decision to pull the US out of a global climate change agreement, a move that infuriated allies far and wide.

The Pentagon's position in recent years had been that climate change presents threats to the nation's security and to stability around the world.