Preventing North Korea from developing nuclear-capable intercontinental missiles is now more-or-less an impossible task, experts say.

But Kim Jong-un would have no intention of starting a war with those nukes, instead using them as a means of holding on to power.

Lowy Institute fellow Thomas Wright said the United States is now running very low on options to freeze or dismantle North Korea's nuclear weapons program .

"There are no good options left and probably no options at all," Dr Wright told nine.com.au.

"Kim Jong-un is determined to developed a ICBM capability and will do so regardless of sanctions or diplomatic pressure."

And a military strike against North Korea would almost certainly result in an all-out war .

"In my view, Kim Jong-un wants the ICBMs (intercontinental ballistic missiles) for two reasons: to guarantee his own survival and to compel the United States to withdrawing or reducing its presence in South Korea ," Dr Wright said.

"He wants the ability to hit the US with nuclear weapons. And he knows that any president would be very hard pressed to prevent him because any war would cause unacceptable loss of life on the allied side."

But the US cannot concede that it is out of options to deal with North Korea , United States Studies Centre research fellow Brendan Thomas-Noone told nine.com.au.

"If you say you don't have an option, you lose any leverage over North Korea," he said.

"That's not how deterrence works."

And while Pyongyang has the capacity to use its existing nuclear weapons, it doesn't have to use them to cause widespread damage.

" Seoul is really close to the North Korean border," Mr Thomas-Noone said.

"(North Korea) wouldn't have to use nuclear weapons, they could just use artillery and regular military force."

It is that arsenal that has kept the US and the South from military action against North Korea for the decades after the Korean War, long before Pyongyang developed the atomic bomb.

And a declaration of war wouldn't even be necessary, with North Korea and the US never actually signing a peace treaty.

But instead, Kim intends to stay in power by creating another Cold War built on mutually assured destruction.

"Their goal is regime survival, and they see the key to regime survival as being able to hold the US mainland at risk of a nuclear strike," Mr Thomas-Noone said.

"And only with that capability do they think the US will never be able to take them out."

North Korea also signalled to the US that they should not attempt airstrikes against missile facilities through test launches over Japan this week.

The latest missile was launched from Pyongyang International Airport, showing North Korea has the capacity to move its missiles around easily.

That would mean the US could not be assured it would successfully destroy all of North Korea's ICBMs if it set out to do so.

It was that uncertainty which prevented John F Kennedy from airstrikes during the Cuban Missile Crisis in 1962.

The lessons of Libya and Iraq, which gave up nuclear weapons programs under incredible international pressure, is one the North Koreans have taken to heart.

"(They think), those are countries that gave up their nuclear weapons program and it could happen to us," Mr Thomas-Noone said.

"The North Koreans think the US could overthrow their regime if they do that."

But the worst case scenario of an actual war "won't happen", says Monash University's Andy Jackson.

"It'll be devastating for that whole area in the centre of the Korean peninsula from Pyongyang down to Seoul," Dr Jackson told nine.com.au.

"There's 30 million people in that area, and that would really seriously affect the world economy. That won't happen. It cannot happen."

Despite having one of the world's biggest armies and the capacity to use nuclear weapons, North Korea would not win a war.

"If it did happen, the North Koreans would lose, and they know that," Dr Jackson said.

"With the South Koreans and the Americans, they'd get flattened."

But even if Kim Jong-un's intent is to keep his nuclear arsenal as an insurance policy, there's a potentially bigger threat.

"There's a real fear in the US that North Korea is exporting nuclear technology to other countries that may want it, say Iran or whoever it would be," Mr Thomas-Noone said.

US President Donald Trump released a strongly-worded statement after North Korea's missile test over Japan.

"Threatening and destabilising actions only increase the North Korean regime’s isolation in the region and among all nations of the world," Trump said.

"All options are on the table."