North Korea's push to acquire a nuclear-armed missile capable of threatening America and other nations is a "clear and present danger", US defence secretary Jim Mattis has said.
Speaking at an international security conference in Singapore, Mr Mattis said the Trump administration was encouraged by China's renewed commitment to working with the US and others to rid North Korea of its nuclear weapons.
He also said he believed China, North Korea's closest ally, will ultimately see Pyongyang as a liability.
China blocked tough new sanctions against North Korea that the United States pushed in the United Nations Security Council on Friday, but the council did vote unanimously to add 15 individuals and four entities linked to Pyonyang's nuclear and missile programmes to a UN sanctions blacklist.
In his speech to the Shangri-La Dialogue, sponsored by the International Institute for Strategic Studies, Mr Mattis sought to balance his hopeful comments on China with sharp criticism of what he called Beijing's disregard for international law by its "indisputable militarisation" of artificial islands in disputed areas of the South China Sea.
"We cannot and will not accept unilateral, coercive changes to the status quo" in the South China Sea, he said.
Overall, Mr 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 US 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."
But he was unrelentingly critical of reclusive North Korea, whose leaders have long viewed the United States as a military threat, in part because of periodic military exercises with South Korea, which the North sees as preparations for attacks aimed at destroying its ruling elite.
The US has about 28,500 troops permanently based in South Korea, a defence treaty ally.
"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," Mr Mattis said, referring to the North's nuclear tests in recent years and an accelerated pace of launches seemingly aimed at building a missile with enough range to hit the US.
"While the North Korean regime has a long record of murder of diplomats, of kidnapping, killing of sailors, and criminal activity, its nuclear weapons programme is a threat to all," he said.
"As a matter of national security, the United States regards the threat from North Korea as a clear and present danger."
Mr 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.
Mr Mattis also used the Shangri-La Dialogue to reiterate his call for international co-operation against violent Islamic extremist groups, such as Islamic State, which he said were trying to gain ground in south-east Asia.
But he made no mention of US president Donald Trump's decision to pull the US out of the Paris global climate change agreement, a move that infuriated allies far and wide.