A US ground invasion of North Korea could cost "millions" of lives, a new military report has predicted amid growing tensions between the US and the hermit nation.

The information has been disclosed after a group of US Congressmen called for an assessment of possible fallout should military action commence.

The study details that the only way that the US could locate and destroy with certainty all components of North Korea's nuclear weapons would be through a ground invasion.

Pentagon official Rear Admiral Michael J Dumont of the Joint Staff revealed that a ground invasion would be the only way to secure the peninsular, sparking concern from Congressmen from both sides of the aisle.

The group of politicians, comprising of 15 Democrats and one Republican, described the assessment as "deeply disturbing" and said such an action "could result in hundreds of thousands, or even millions of deaths in just the first few days of fighting."

Rear Admiral Michael J Dumont, the vice director of the Pentagon's Joint Staff, wrote the letter in response to a request from two House members about "expected casualty assessments in a conflict with North Korea," including for civilians and US and allied forces in South Korea, Japan and Guam.

"A decision to attack or invade another country will have ramifications for our troops and taxpayers, as well as the region, for decades," Ted Lieu (Democrat, California) and Ruben Gallego (Democrat, Arizona) wrote to the Pentagon. "We have not heard detailed analysis of expected US or allied force casualties, expected civilian casualties, what plans exist for the aftermath of a strike — including continuity of the South Korean Government."

Senior Democrat Diane Feinstein, a member of the Senate Intelligence Committee, gave a downbeat view on CNN. "I've spent a lot of time reading the intelligence. I've had an opportunity to discuss the situation with [Defense] Secretary [James] Mattis. I believe that an outbreak of war would kill hundreds of thousands of people."

The new findings come just as Donald Trump embarks on his first tour of Asia which is seeing him visit South Korea.

Speaking from an airbase in Tokyo on Sunday, Trump said that "no one, no dictator, no regime ... should underestimate American resolve".

The Congressman raised doubts about the ways in which Trump has dealt with the growing threat from the Kim regime.

The Trump administration "has failed to articulate any plans to prevent the military conflict from expanding beyond the Korean peninsula and to manage what happens after the conflict is over".

"With that in mind, the thought of sending troops into harm's way and expending resources on another potentially unwinnable war is chilling. The president needs to stop making provocative statements that hinder diplomatic options and put American troops further at risk," the lawmakers said.

Trump has taken to Twitter using strong verbal threats to dissuade Pyongyang from their continued efforts to produce a fully-functioning nuclear powered inter-continental ballistic missile.

The president has mocked his counterpart, calling him "rocket man" during his maiden UN speech and threatening the nation with "fire and fury".

It comes after a series of missile and nuclear tests in the past few months, a sharp escalation from recent years.

Donald Trump and North Korea
US President Donald Trump departs after a round of golf with Japan's Prime Minister Shinzo Abe at Kasumigaseki Country Club in Kawagoe, Japan Jonathan Ernst/Reuters