N. Korea slams 'incurably mentally deranged' Trump

North Korea slammed US President Donald Trump as "incurably mentally deranged" in a personal attack ahead of his first visit to Asia, as the South's leader insisted Washington could not take military action on the peninsula without his agreement.

Trump and the North's leader Kim Jong-Un have traded threats of war and personal insults against each other in recent months, heightening worries about another conflict on the peninsula where the 1950-53 Korean War left millions dead.

The US leader has warned of "fire and fury", telling the UN General Assembly that Washington would "totally destroy North Korea" if it had to defend itself or its allies.

Washington and Seoul have been in a security alliance for decades, and the US has 28,500 troops stationed in the South to defend it from the North.

Trump dubbed Kim "Rocket Man" in the same speech -- Pyongyang has tested missiles apparently capable of reaching much of the US mainland -- and days later Kim responded with a personal statement calling him a "dotard", an obscure term for a weak or senile old man.

The US president is due in Asia at the weekend and ahead of his arrival the North's state-run KCNA news agency lashed out at "bellicose and irresponsible rhetoric" by the "master of invective".

Washington has deployed key military assets including jet fighters and aircraft carriers near the peninsula following the North's sixth nuclear test in September, which also saw the United Nations impose an eighth set of sanctions on the isolated country. 

Trump, KCNA said late Tuesday, "disclosed his true nature as a nuclear war maniac before the world and was diagnosed as 'incurably mentally deranged'".

The North has a long history of colourful personal attacks against US leaders. It has called Trump's predecessors Barack Obama and George W. Bush a "monkey" and "half-baked man".

It has railed against former South Korean president Park Geun-Hye as a  "witch" and a "crafty prostitute" who had Obama as her "pimp".

- 'Tragic history' -

Despite the fire-breathing rhetoric, South Korean President Moon Jae-In on Wednesday said his country would not develop or possess nuclear weapons, unlike its neighbour. 

South Korean media and opposition politicians have called for the return of US tactical nuclear weapons, which were withdrawn from the peninsula in the 1990s.

Some have suggested that if Washington does not agree -- Defense Secretary Jim Mattis expressed doubts about the prospective move at the weekend -- Seoul should develop a nuclear capability of its own, in order to ensure a so-called "balance of terror" on the peninsula.

In his address to the National Assembly Moon, who has advocated engagement to bring the North to the negotiating table, insisted: "There should be no military action on the peninsula without our prior consent."

The fate of the flashpoint region should be determined by Koreans, he added, saying the country should "not repeat the tragic history" of colonisation and division.

The Korea peninsula was controlled by Japan from 1910 to 1945, and after Tokyo's surrender ended the Second World War it was divided into separate zones of occupation by Russia and the US. 

The latest standoff has heightened concerns among South Koreans, who have over decades grown indifferent to regular threats of attack from Pyongyang.  

But even some Trump advisers say US military options are limited because any armed conflict on the peninsula would be expected to cause huge casualties. 

Seoul is home to 10 million people and only about 50 kilometres (30 miles) from the heavily-fortified border, within range of Pyongyang's massive artillery forces.

Pyongyang hails its nuclear arsenal as a "treasured sword" to protect itself from potential invasion by its "imperialist enemy" the US, but has threatened to bracket the US Pacific island of Guam with missiles.

Trump's itinerary includes Japan, South Korea, China, Vietnam and the Philippines, with all eyes on his message to the North and Kim.

In the South Trump is due to address parliament, visit a US military base, and hold a summit with Moon, although he will not go to the Demilitarized Zone that divides the two Koreas.