Mystery surrounds the death of Kim Jong-nam after he was poisoned in a Cold War-style assassination last month, but an intriguing conspiracy theory has thickened the plot even further.

North Korea has been blamed for the death as Kim Jong-nam, who was a potential rival to his half brother and the country's leader Kim Jong-un.

But one puzzling aspect of the situation was that Kim Jong-nam was seen to be under the protection of China, and killing him would cause problems with North Korea's only real ally.

In fact China has halted all coal imports from North Korea since the death, which some believe is a sign of Beijing's displeasure.

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There has been speculation the killing may have infuriated Beijing because Kim Jong-nam was considered a pro-Chinese candidate to replace Kim Jong-un if the current government fell.

Did China allow the killing?

But Brad Glosserman, executive director of the Center for Strategic and International Studies in the US, told news.com.au there was an interesting conspiracy theory doing the rounds that China let the assassination happen..

"If Kim Jong-un didn't kill this guy, or he wasn't killed on his orders, you've killed the half brother of the king," Glosserman said.

"Unless the king thinks that's a good idea, you've run one hell of a risk."

He said North Korea may have been allowed to assassinate Kim Jong-nam as some kind of elaborate "signal" from Beijing to Pyongyang.

North Korean leader Kim Jong-un. Photo / AP
North Korean leader Kim Jong-un. Photo / AP

"Because in theory he was under their protection, the Chinese [may have] let that assassination occur, as a way of signalling to Pyongyang that even if [they] are moving forward with the sanctions on the coal exports and tightening the screws, this is proof that [the Chinese are] not really out there to overthrow the regime," Glosserman said.

Since the assassination, China has acted in a way that suggests it's not happy with its neighbour, announcing it will no longer import coal from North Korea and that it will work more closely with the US over concerns about North Korea's nuclear programme.

But China's crackdown may turn out to be just for appearance's sake.

Glosserman said it was unclear whether China's halting of coal imports was doing any real damage to North Korea.

Glosserman said China may have already bought its year's allocation of coal so it was unclear whether it would have bought much more anyway. There's also been suggestions China may be offsetting its coal purchases through the use of liquid natural gas, and the border between the two countries is quite porous so it's difficult to know how strictly the sanctions are being enforced.

But China has been under pressure to act, with US President Donald Trump even tweeting that it had done "little to help" keep North Korea from behaving badly.

Although China has now agreed to work more closely with the US on addressing North Korea's nuclear ambitions, Glosserman said this would probably have little effect.

"There's no agreement between the US and China. All [they] agree on is 'this is bad' [and] that North Korea should give up its nuclear weapons," he said.

Beyond that there was no agreement on how to achieve this.

What happens if the North goes nuclear?

Although everyone wants a diplomatic solution in North Korea, Glosserman said he thought it was unlikely a deal could be achieved.

"North Korea is the land of bad options. There are no good choices because North Korea believes a nuclear weapon is the key to the survival of the regime, not just North Korea as a separate government, but to the Kim regime," he said.

However, if the US did become open to the idea of North Korea having nuclear weapons, Glosserman said it would create "profound stresses" in its relationship with South Korea and Japan, as well as undermining the integrity of the non-proliferation treaty.

North Korea having nuclear weapons create
North Korea having nuclear weapons create "profound stresses" in its relationship with South Korea and Japan. Photo / AP

Perhaps even more worryingly, it could start a domino effect.

"I believe North Korea went nuclear because Pakistan went nuclear and got away with it. And I'm willing to bet that if North Korea goes nuclear, Iran will go nuclear and if Iran goes nuclear who knows what other dominoes will fall?," Glosserman said.

"If North Korea is allowed to become a nuclear weapons state, I would suggest South Koreans might be encouraged to do the same and the Japanese will actively be pushed to do the same.

"There are a number of nuclear dominoes that have the potential to fall."

Trump administration could make 'huge mistake'

The election of Donald Trump as US President also seems to be throwing up the worrying prospect of a world with even more nuclear weapons.

The Trump administration appears to be leaving open the possibility of allowing more nuclear weapons, after Secretary of State Rex Tillerson said this week that "circumstances could evolve" in terms of Tokyo acquiring atomic weapons to counter the North Korean threat.

"We need to have fewer nuclear weapons, not more, but that doesn't seem to be the view of the State Department or this administration," Glosserman said.

United States President Donald Trump. Photo / AP
United States President Donald Trump. Photo / AP

"If this US administration is kind of encouraging, as Tillerson seems to have hinted in his comments in Tokyo, that maybe that's a path worth taking, I think that's a huge mistake."

However, Glosserman believes South Korea's elections in May will be important and may change the dynamics in the region if a more progressive leader, who embraces North Korea, is elected.

"Suddenly the US bargaining position is diminished," he said. "Because suddenly the North Koreans can get by. The problem is, over time, they acquire more capabilities."

Glosserman said experts had developed "assumptions" to guide the way they thought about North Korea, but unfortunately they didn't produce "very happy outcomes".

Generally experts were "floundering" when it came to predicting how the North Koreans would behave.

"It's extraordinary that having studied North Korea for as long as we have, how little we know about the actual workings of the decision-making body," Glosserman said.

"If anyone tells you they know what the North Koreans are going to do, they're lying."