North Korea's sixth and largest-yet nuclear test, a detonation that occurred while most Americans dozed during the long weekend, may have finally ended efforts to prevent Pyongyang from achieving nuclear weapons capable of reaching the U.S. mainland, experts say.

The North now claims to have a hydrogen bomb. It has at the very least blown up a thermonuclear weapon that, if mounted on an intercontinental ballistic missile, could devastate an American city.

If that's indeed the case, experts say, the international community must shift focus away from keeping such weapons out of the North's hands to making sure the rogue state never deploys them.

Sunday's massive explosion caused a jolt that registered a 6.3 on the Richter scale, a seismic event that experts say could indicate a hydrogen bomb. Such a device would be eight times more powerful than the bomb dropped on Hiroshima during the Second World War.

Nuclear War Anxiety 20170810

A man watches a television screen showing U.S. President Donald Trump, left, and North Korean leader Kim Jong-un during a news program at the Seoul Train Station in Seoul last month. (Ahn Young-joon/Associated Press)

"Once they get to a hydrogen bomb level, we're talking millions of tonnes of TNT," said Harold Kazianis, director of defence studies at the Center for the National Interest, a Washington think-tank.

"We're out of time when it comes to stopping North Korea in acquiring nuclear weapons. They already have them, they're already there."

Making the option of using such weapons unthinkable or at least extremely unappealing for North Korea could come down to more deterrence through military exercises and shows of regional co-ordinated military strength, experts say.

U.S. President Donald Trump and Treasury Secretary Steve Mnuchin are also telegraphing policies of containment to cut off the North Koreans from the economic, diplomatic and financial systems of the world.

No good options

While there are never good options when dealing with North Korea, Kazianis said, "it's time to focus on the here and now" by applying tougher sanctions on the North's allies and ratcheting up diplomatic pressures.

He believes the North may be only six to 18 months away from a key step of miniaturizing and mounting a hydrogen bomb on a missile that can reach the U.S. mainland.

Whatever the case, Kazianis said, the U.S. response shouldn't veer into a unilateral military option that would trigger catastrophic collateral damage. 

APTOPIX South Korea Koreas Tensions

In this photo provided by South Korea Defence Ministry, South Korea's Hyunmoo II ballistic missile is fired during an exercise at an undisclosed location in South Korea on Monday. The South Korean military said it conducted a live-fire exercise simulating an attack on North Korea's nuclear test site to 'strongly warn' Pyongyang over the latest nuclear test. (South Korea Defence Ministry/Associated Press)

A targeted strike on nuclear weapons sites, for example, risks triggering war in the Korean Peninsula if the U.S. can't guarantee 100 per cent accuracy on wiping out the entire nuclear program, Kazianis said.

"The challenge is we don't know where all the North Korean nuclear weapons are," Kazianis said. "Say you take out 28 out of 30 nuclear weapons, if Kim has one or two left, he'll launch them at South Korea or Japan or the west coast of the United States."

Aside from its atomic arsenal, North Korea still has 10,000 artillery tubes pointed at South Korea and 4,500 tonnes of chemical weapons "so you'd still be condemning hundreds of thousands of people to die," Kazianis said.

When a reporter asked Trump on Sunday if he would attack the North, Trump answered, "We'll see."

Military threat

U.S. Defence Secretary James Mattis struck a strident tone in discussing the prospect of war in the Korean Peninsula.

"Total annihilation" of a country, Mattis said, "namely North Korea," was at risk if it continued its aggression against the U.S. or its allies.

Experts agree that a military threat from the U.S. remains the most compelling reason for why North Korean leader Kim Jong-un is likely to hold back from approaching all-out war.

'Kim understands an attack on the U.S. homeland would likely lead to the end of their regime.' - Anthony Ruggiero

"The North Koreans aren't suicidal," said Anthony Ruggiero, a senior fellow with the Foundation for Defence of Democracies, a Washington think-tank. "Kim understands an attack on the U.S. homeland would likely lead to the end of their regime."

But by declaring it has obtained a hydrogen bomb and with its stepped-up tests of its intercontinental ballistic missile, the North Korean leader is also gambling that the U.S. would be hesitant to risk the lives of its own citizens by intervening in the North's ultimate goal of reunifying the Korean Peninsula, Ruggiero said.

"For them, this is all about having the capability to attack the U.S. homeland," he said. "Does North Korea sort of wrap this up and say, 'We showed you the H-bomb, we conducted two ICBM tests, now we're going to declare it's on an ICBM that we're willing to use on the U.S.?' "

Likely Pyongyang now believes it may have reached a point at which it's too late for the U.S. to try to take away its atomic weapons, he said.

With the rapid pace at which the North has accelerated its weapons program, Ruggeiro said Washington can't afford to wait much longer for the United Nations in order to press ahead with the most robust sanctions it can.

Sanctions proposed

"I think the U.S. can impose sanctions the first day of business this week, on Tuesday morning, if they really wanted to, and show the Chinese and the Russians we're serious."

Congress returns from a summer break on Tuesday and Mnuchin is drafting a sanctions package to present to the president.

He told Fox News the package would indicate "that anybody that wants to do trade or business with [North Korea] would be prevented from doing trade or business with us," a pointed reference to the North's allies China and Russia.

Beijing continues to export crude oil and food to Pyongyang, but the U.S. cutting off business with the Chinese could cause profound economic pain. The U.S. and China do more than $600 billion in annual trade.

APTOPIX China Koreas Tensions

Trucks cross the bridge connecting China and North Korea in the Chinese border town of Dandong, opposite side of the North Korean town of Sinuiju on Sept. 4, 2017. China said Monday that Trump's threat to cut off trade with countries that deal with North Korea is unacceptable and unfair. (Helene Franchineau/Associated Press)

Decades of dithering in Washington about what to do about North Korea have led up to this moment, said former State Department adviser David Asher, who headed a task force into North Korea during the George W. Bush administration.

"Where on earth is the North Korean regime getting the money for all this nuclear activity? Their GDP is eclipsed by these programs, so obviously they're getting outside assistance."

Asher would like to see sanctions targeting Chinese and Russian firms and individuals.

"Whether we're going to do it, I don't know. We'll see if we have the gumption to do it," he said. "I'm a diplomat and I'm not against diplomacy, but I'm also a realist. We can't stop living in a surrealistic strategy where the clocks have melted like a Salvador Dali painting. Time hasn't stood still."