South Korea's President Moon Jae-in has ruled out a "war again on the Korean Peninsula" and is considering sending a special envoy to North Korea for talks if Pyongyang stops its missile and nuclear tests.

Moon's comments on Thursday, marking his 100 days in office, come amid increased tensions between the United States and North Korea following Pyongyang's warning that it might send missiles into waters near the US territory of Guam.

That was followed by US President Donald Trump's threats to unleash "fire and fury" on North Korea.

"The people worked together to rebuild the country from the Korean War and we cannot lose everything again because of a war," Moon said in a nationally televised news conference.

"I can confidently say there will not be war again on the Korean Peninsula."

In recent days, both Koreas and the US have signalled a willingness to avert a deepening crisis, with each suggesting a path towards negotiations.

Earlier this month, the UN adopted tough new sanctions against North Korea after it launched its second intercontinental ballistic missile test (ICBM) in July.

Moon said a "red line" would be Pyongyang "completing its ICBM and mounting it with a nuclear warhead and weaponising it".

"If North Korea launches another provocation, it will face even stronger sanctions and it will not be able to survive them. I would like to warn North Korea to end its dangerous gamble."

'Dialogue must resume'

The South Korean leader was elected in May after a decade of conservative rule that saw animosity deepen between the rival Koreas.

Moon has repeatedly said he wants to engage with the North, but his efforts have so far met a string of threats and missile tests as North Korea's leader Kim Jong-un works to build nuclear-armed missiles that can reach the US mainland.

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"A dialogue between South and North Korea must resume. But we don't need to be impatient," Moon said.

"I think lots of effort and time could be necessary to overcome a decade of severed ties and to reopen a dialogue."

Moon added Washington and Seoul are closely communicating over the North Korean nuclear programme and share the view that strong sanctions are needed against Pyongyang to stop its provocations and force it into negotiations.

Moon said he thinks Trump's belligerent words are intended to show a strong resolve for pressuring North Korea and don't necessarily display the willingness for military attacks.

"The United States and President Trump have already promised to sufficiently consult with South Korea and get our approval for whatever option they will take against North Korea. It's a firm agreement between South Korea and the United States," Moon said.

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On Wednesday, the UN Secretary-General Antonio Guterres said it was time to "dial down rhetoric and dail up diplomacy" on North Korea, offering to help broker talks with the parties involved in the dispute.

"The solution to this crisis must be political. The potential consequences of military action are too horrific to even contemplate," Guterres said.

The US and South Korea are set to begin their annual military drills next week, which enrage North Korea each year.