The Asia-Pacific region must not be "held hostage to a dictator's twisted fantasies of violent conquest and nuclear blackmail," Donald Trump has said.
The US President warned "every step the North Korea regime takes towards more weapons is a step it takes into greater and greater danger".
His latest tough talk on Pyongyang and its leader Kim Jong-un came during a speech at Asia-Pacific Economic Cooperation summit in Danang, Vietnam.
The issue of North Korea's nuclear weapons programme has loomed large over Mr Trump's first tour of Asia.
The President is looking to win support for ramping up sanctions on Pyongyang during visits to Japan, South Korea, China, Vietnam and the Philippines.
His speech came as South Korea announced it would join three US aircraft carrier strike in a rare military drill in the Western Pacific over the weekend.
The USS Ronald Reagan, the USS Theodore Roosevelt and the USS Nimitz will sail together for the first time in a display of military might timed to coincide with the last leg of the President's tour and amid heightened tensions in the region over North Korea.
Addressing an audience of chief executives on Friday, Mr Trump also claimed the US had not been "treated fairly by the World Trade Organisation" and said he would not let his country be "taken advantage of anymore".
In what appeared to a rebuke of China, he said America has lowered market barriers but "other countries didn't open their market to us".
"From this day forward we will compete on a fair and equal basis," he said, hours after leaving Beijing.
"We are not going to let the United States be taken advantage of anymore. I am always going to put America first.
"We can no longer tolerate these chronic trade abuses and we will not tolerate them."
It was a striking change of tone from Thursday, when Mr Trump set aside his previous blistering rhetoric in favour of friendly overtures to China as he sought to flatter his host into establishing a more balanced trade relationship.
During the presidential election campaign, Mr Trump condemned China for what he claimed were inappropriate trade practices.
By contrast, in Beijing this week he criticised the "very one-sided and unfair" relationship between the US and China but said he did not blame the Chinese for having taken advantage of the US in the past.
He said China "must immediately address the unfair trade practices" that drive a "shockingly" large trade deficit, along with barriers to market access, forced technology transfers and intellectual property theft.
"But I don't blame China," he said. "After all, who can blame a country for being able to take advantage of another country for the benefit of its citizens?"
To applause, he said: "I give China great credit."
China's trade surplus with the US for the first 10 months of this year was $223bn (£170bn).
In Friday's speech, Mr Trump also claimed "the whole world" was being "lifted by America's economic renewal" and that a "new optimism" has swept across the US since his election.
He said he had had the pleasure of sharing the "good news from America" at every stop on his first official visit to Asia.