North Korea has responded to Donald Trump's threat to destroy the reclusive nation by announcing it may test a hydrogen bomb over the Pacific Ocean.
Leader Kim Jong-un did not specify what action he would take against the US or Mr Trump, whom he called a "mentally deranged US dotard" in the latest bout of insults the two leaders have traded in recent weeks.
South Korea said it was the first direct statement of its kind by a North Korean leader.
However, Kim's foreign minister, Ri Yong-ho, said North Korea could consider a hydrogen bomb test of an unprecedented scale over the Pacific Ocean.
Mr Ri told reporters in New York he did not know Mr Kim's exact thoughts.
Japan, the only country ever to suffer an atomic attack, described the threat as "totally unacceptable".
The US President, who has not shrunk from fighting fire with fire in his rhetoric on North Korea, sent another message Friday on Twitter.
"Kim Jong-un of North Korea, who is obviously a madman who doesn't mind starving or killing his people, will be tested like never before," Mr Trump said, a day after announcing additional sanctions on Pyongyang.
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Mr Trump said in his first address to the United Nations on Tuesday he would "totally destroy" North Korea, a country of 26 million people, if it threatened the United States and its allies, and called Mr Kim a "rocket man" on a suicide mission.
Mr Kim said the North would consider the "highest level of hard-line counter-measure in history" against the United States and that Mr Trump's comments had confirmed his own nuclear program was "the correct path".
Pyongyang conducted its sixth and largest nuclear test on September 3 and has launched dozens of missiles this year as it accelerates a program aimed at enabling it to target the United States with a nuclear-tipped missile.
"I will surely and definitely tame the mentally deranged US dotard with fire," Mr Kim said in the statement on the KCNA state news agency.
Asked about the North Korean hydrogen bomb threat, Secretary of State Rex Tillerson told ABC that diplomatic efforts will continue but all military options were still on the table.
A senior US official said on Friday it would be a "game-changer" if North Korea conducts a hydrogen bomb test over the Pacific and the United States is taking the threat seriously, but it does not give "too much credence" to Pyongyang taking such action.
The official, speaking on condition of anonymity, said there were questions whether North Korea had the capabilities to conduct such a test but declined to specify how the United States would respond.
Mr Trump and South Korean President Moon Jae-in committed to strengthen their "combined defence posture, including through South Korea's acquisition and development of highly advanced military assets" and "agreed to the enhanced deployment of US strategic assets in and around South Korea on a rotational basis", the White House said in a statement.