SEOUL (BLOOMBERG) - As US President Donald Trump threatens to destroy North Korea, even some of his closest aides have warned of the potentially disastrous effects of a war.

New research published on the 38 North website points to just how catastrophic the impact might be on the regime's neighbours.

If North Korean leader Kim Jong Un were to launch a nuclear attack on Seoul and Tokyo - both within striking distance of his weapons - as many as 2.1 million people could die and another 7.7 million could be injured, according to the 38 North report.

The analysis by Mr Michael Zagurek Jr, a consultant specialising in databases and computer modelling, is based on North Korea's current estimated weapons technology and bomb strength.

Mr Zagurek assumes Mr Kim has a baseline arsenal of 20-25 warheads and the capacity to put them on ballistic missiles.

Concerns about a nuclear conflict in North Asia have increased as Mr Kim accelerates his programme of acquiring weapons capable of hitting the continental United States, and as Mr Trump threatens pre-emptive military action.

While the chance of a direct attack on US allies Japan and South Korea remains slim, Mr Zagurek said history was replete with miscalculation by "rational actors" during crisis situations.

North Korea's Foreign Minister in September said the regime's possible next steps include testing a hydrogen bomb over the Pacific Ocean.

North Korea's older warheads have yields in the 15-25 kiloton range, around the size of the bombs that devastated Hiroshima and Nagasaki in 1945.

Fatality estimates rise significantly if North Korea were able to strike with bombs similar to the one it tested on Sept 3, which had a likely yield of 108-205 kilotons, Mr Zagurek said.