A war with North Korea would result in the "end of humanity", the Philippine president warned, in the strongest comments yet against a military confrontation after US President Donald Trump wrapped up his Asia-Pacific trip.

Rodrigo Duterte predicted a war with the North would end in a "nuclear holocaust" in his closing remarks at the ASEAN summit on Tuesday in Manila.

He said North Korean leader Kim Jong-un "is toying with these nuclear bombs" that are 200-300 times more powerful than what the United States dropped on Japan during World War II.

''We cannot start a war with the North Korean crisis looming ahead. There are dark clouds there. We better pray," Duterte told the audience.

"If all of those missiles and the (intercontinental ballistic missile) ICBM's would explode, that would mean the end of humanity… The destruction, it would be the end of everything," he warned.

The Philippine leader's comments came after a 12-day tour of the region by Trump with discussions on how to neutralise the North's nuclear weapons programme high on the agenda along the way.

Coinciding with the US president's visit, South Korean warships and three American aircraft carrier strike groups converged off the Korean Peninsula for major war exercises.

North Korean state media described the unusual military build-up as a direct threat against the communist country.

The US "is now driving the peninsula situation to the worst phase of nuclear war as it finds fault with the DPRK's self-defensive countermeasures to boost its nuclear capability", the weekly Pyongyang Times newspaper said in a Tuesday commentary headlined "Nuclear clouds gather over Asia-Pacific."

"At present, the US is taking extremely risky military actions. Seven out of its 11 aircraft carriers are fanning out simultaneously to the operational theatre close to the peninsula and F-35C latest stealth fighter has been fielded to the region for the first time," it added.

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North Korea has launched a series of international ballistic missile (ICBM) tests in recent months, and detonated its sixth and most powerful nuclear device in September.

Military analysts have suggested Pyongyang will soon acquire the technological capability to launch an ICBM with a nuclear warhead that can strike the mainland US.

The US has said it will never accept a nuclear-armed North Korea, and Trump has threatened to "totally destroy" the country if it threatens the US or its allies.

"A rogue regime that threatens the world with nuclear devastation has no place in our community of sovereign nations," Trump said in departing remarks on Tuesday.

Meanwhile, South Korea's President Moon Jae-in suggested discussions among allies on a quid pro quo for North Korea if it comes to the negotiating table and freezes its nuclear programme.

"I believe it will not be easy realistically to move on to complete dismantlement of North Korean nukes in the near future, considering recent advances in North Korea's nuclear and missile programmes," Moon said at a press conference on Tuesday.

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"That means it will likely be North Korea first freezing its nuclear programme and then moving onto complete dismantlement. And if that happens, I believe we and the international community may discuss what we can do in return," the Yonhap news agency quoted Moon as saying.

North Korea has vowed it will never abandon its nuclear capabilities, calling the weapons a deterrent against "invasion and plunder" by the US.