Up to 20,000 people would be killed in South Korea each day in a war between the United States and North Korea, a retired US Air Force brigadier general has said.

“Too many Americans have the view that [war with North Korea] would be like the invasion of Iraq or Afghanistan, or like combat operations in Libya or Syria, but it wouldn’t remotely resemble that,’’ Rob Givens, who spent four years stationed on the Korean peninsula, told the Los Angeles Times.

Such a conflict risks a devastating artillery barrage on the South Korean capital city of Seoul.

The Pentagon estimates as many as 20,000 people a day could be killed in South Korea, Mr Givens said.

Trump: US's military solution for North Korea would be 'devastating'

James Stavridis, a retired Navy admiral, told the paper the chances of a conventional conflict with North Korea were 50/50.

He said war could be sparked by North Korean leader Kim Jong-un launching a missile which lands on or near the US island territory of Guam.

US aircraft carriers would then retaliate with an air stroke on a North Korean coastal launch facility.

North Korea would likely respond by using its 11,000 artillery units to target the 35,000 US troops currently stationed in North Korea, eventually firing on Seoul's 25 million inhabitants as the conflict escalated.

Kim Jong-un inspects weapon North Korea says is powerful hydrogen bomb Show all 6 1 /6 Kim Jong-un inspects weapon North Korea says is powerful hydrogen bomb Kim Jong-un inspects weapon North Korea says is powerful hydrogen bomb Photos released by North Korea show Kim Jong-un talking to subordinates next to a device thought to be the new thermonuclear weapon. There is no way of independently verifying the pictures STR/AFP/Getty Images Kim Jong-un inspects weapon North Korea says is powerful hydrogen bomb North Korea claims it has successfully tested an advanced hydrogen bomb which could be loaded onto an intercontinental ballistic missile AFP/Getty Kim Jong-un inspects weapon North Korea says is powerful hydrogen bomb A diagram on the wall behind Mr Kim shows a bomb mounted inside a cone STR/AFP/Getty Images Kim Jong-un inspects weapon North Korea says is powerful hydrogen bomb North Korean leader Kim Jong-Un (C) attending a photo session with participants of the fourth conference of active secretaries of primary organisations of the youth league of the Korean People's Army (KPA) in Pyongyang STR/AFP/Getty Images Kim Jong-un inspects weapon North Korea says is powerful hydrogen bomb A new stamp issued in commemoration of the successful second test launch of the "Hwasong-14" intercontinental ballistic missile KCNA via Reuters Kim Jong-un inspects weapon North Korea says is powerful hydrogen bomb A new stamp issued in commemoration of the successful second test launch of the "Hwasong-14" intercontinental ballistic missile KCNA via Reuters

The US Department of Defence has conducted military simulations to find "four or five ways to resolve" a conflict with North Korea, according to National Security Adviser H R McMaster.

"And some are uglier than others," he said at a Washington event hosted by the Institute for the Study of War.

“There’s not a ‘precision strike’ that solves the problem,” Mr McMaster said.

“There’s not a military blockade that can solve the problem. What we hope to do is avoid war, but we cannot discount that possibility.”

North Korean leader Kim Jong Un delivers a statement in response to Donald Trump's speech to the United Nations (AP)

Donald Trump has warned the US is ready with a "military option" to end the crisis with North Korea.

"We are totally prepared for the second option, not a preferred option," the US President said.

"But if we take that option, it will be devastating, I can tell you that, devastating for North Korea. That's called the military option. If we have to take it, we will."

The White House dismissed North Korea's suggestion Mr Trump had "declared war" on Pyongyang as "abusrd."

North Korea's foreign minister Ri Yong-ho said Mr Trump's comments, in which he said the foreign minister and North Korea's leader "won't be around much longer" if they continued their threats, amounted to a declaration of war.