Seoul - On the surface it seems like more of the same: North Korea responds to another threat by US President Donald Trump by calling him a "deranged" old man who will "pay dearly" for his insults.These words, however, carry the weight of an unprecedented personal rebuke from North Korea's supreme leader, Kim Jong Un.Behind Kim's colourful language released on Friday is a remarkable window into the thinking - albeit filtered through state media - of the leader of a country that, despite opposition by the world's sole superpower, stands on the brink of nuclear weapons mastery.Here are five things to know about Kim Jong Un's statement:***HE'S BREAKING GROUNDIt was written in the first person, and issued directly to the international community generally and to Trump specifically.Seoul's Unification Ministry, which is responsible for North-South relations, said it was the first time a North Korean leader had addressed the world with such a direct statement.The ministry said neither of the two men to rule North Korea before Kim Jong Un - his father, Kim Jong Il, and his grandfather, national founder Kim Il Sung - issued any similar statements.It could be that Kim felt that a direct, personal response was crucial because of the harshness of Trump's comments. The US president vowed to unleash "fire and fury" on North Korea, used the nickname "Rocket Man" for Kim, and at the UN on Tuesday threatened to "totally destroy" the North if provoked.***HE'S ISSUING A WARNINGThe statement suggests more powerful weapons tests are in the works.North Korea's foreign minister seemed to confirm this on the sidelines of a global UN meeting in New York, telling reporters that Kim's comments could mean that North Korea will conduct an H-bomb test in the Pacific, according to South Korea's Yonhap news agency.Ri Yong Ho added that no one knows for sure, and Kim would make any decisions on such a test, which would be viewed as massively provocative by Washington.Analysts in Seoul also saw the statement as a warning that more tests, possibly of the country's developmental intercontinental ballistic missiles, should be expected.***HE'S PLAYING THE STATESMANBelieve it or not, Kim's statement actually used gentler language than his propaganda specialists have favored in the past.Granted, he called Trump a "mentally deranged US dotard" (a word to describe a fragile elderly person) and a "frightened dog".But this is a far cry from North Korea at its worst.North Korea has previously embraced racist, sexist and just plain rude statements about its enemies. It repeatedly called one past South Korean leader "rat faced". It has said that the South's first female president, Park Geun-hye, was a "crafty prostitute" and suggested ex-President Barack Obama was her pimp. It also called Obama a "monkey".Trump will surely not appreciate the language, but it's a far cry from North Korean anger at full force. That suggests Kim could be trying to take a more statesmanlike path than his state media.***HE FEELS JUSTIFIEDKim says Trump's threats only emphasise that North Korea has been justified in its pursuit of nuclear missiles.North Korea has long said that its weapons tests are necessary because of US hostility, which for Pyongyang includes the nearly 80 000 US troops stationed in Japan and South Korea.Each Trump threat plays into this narrative.Here's the key line in Kim's argument: "His [Trump's] remarks which described the US option through straightforward expression of his will have convinced me, rather than frightening or stopping me, that the path I chose is correct and that it is the one I have to follow to the last."***HE'S INSULTEDKim seemed to take umbrage that Trump was personally insulting him. After all, he typically leaves the threats to his propaganda mavens and lower-level officials.Kim essentially says that he expected better of Trump. Because the US president was speaking publicly on the world stage at the UN, Kim thought he'd resort to "stereo-typed, prepared remarks a little different from what he used to utter in his office on the spur of the moment"."But, far from making remarks of any persuasive power that can be viewed to be helpful to defusing tension, he made unprecedented rude nonsense one has never heard from any of his predecessors," Kim continued.Kim offered some unsolicited advice to Trump as well. He advised the president "to exercise prudence in selecting words and to be considerate of whom he speaks to when making a speech in front of the world".He added that "Trump has denied the existence of and insulted me and my country in front of the eyes of the world."In a country where Kim's word is law, the message seems clear: This will not stand.