Image copyright AFP Image caption The apparent killing of Kim Jong-nam raises tricky questions for China

Beijing needs to do more to rein in North Korea: that's the view of US President Donald Trump and his new team. But how much leverage does China really have there and what are the chances of it being used, asks the BBC's Stephen McDonell in Beijing.

China and North Korea seem to be heading into yet another tense period in their recently rocky relationship.

Once brothers-in-arms fighting against "imperialist aggression" during the Korean War, now Beijing accuses Pyongyang publicly of breaching United Nations sanctions in the pursuit of its missile and nuclear weapons programmes.

And the apparent assassination of Kim Jong-nam - the half brother of North Korea's brutal leader - is being seen as a fresh point of tension between these official allies.

In fact, some view it as direct slap in the face for China.

It appears Mr Kim was murdered in Kuala Lumpur airport, on his way back to Macau, by female killers using of some type of poison.

Image copyright AFP Image caption Kim Jong-nam died at Kuala Lumpur airport as he prepared to board a flight

Kim Jong-nam spent much of the past decade in a type of self-imposed exile inside the former Portuguese colony. There he was seen to have the protection of China.

The eldest son of North Korea's late leader Kim Jong-il, he said time and again that he had no interest in becoming involved in his country's politics.

What's more, whenever he was cornered by reporters in the Asian casino city, with his shirt unbuttoned to number three and sporting a three-day growth, you could really believe him when he said it. After all, why would he want to?

There has been speculation that he operated some sort of North Korean sanction-busting slush fund out of Macau and that this was the reason that Beijing and Pyongyang tolerated his hedonistic life style.

But for China there was something else too. He was an ally inside the North Korean elite: somebody who thought the best way forward for his homeland was a Chinese-style opening up.

Substitute leader?

For years, China has been trying to promote this style of thinking with its isolated, impoverished neighbour.

Before he died, Kim Jong-il was shown around the prosperous Chinese city of Dalian. The message: "You too could have some of this at home with a bit of opening up!"

But the Kim dynasty has appeared petrified by the prospect of such openness, and that Kim Jong-nam would side with the Chinese.

So despite his apparent lack of interest in political power, the fact that he could be seen hanging around down in Macau as a possible leader to be called on by Beijing in the event of regime collapse in Pyongyang made him a threat to the paranoid figure in power there today.

If this was a political assassination, then most North Korea observers think the order came right from the top.

Image copyright AFP/KCNA Image caption China joined UN Security Council condemnation of North Korea's latest missile test

This will not go down well with the government of Xi Jinping in Beijing. In recent days the two countries' relationship has become even more murky.

South Korea's Yonhap news agency has reported that China turned back a $1m (£800,000) coal shipment from North Korea.

China has long been criticised for turning a blind eye to North Korean coal exports, in violation of UN sanctions, but maybe not this time.

In the wake of last weekend's North Korean ballistic missile test, 16,295 tonnes of its coal were denied entry to Wenzhou Port in Zhejiang Province.

You see the sequence of events: Sunday 12 February missile test, next morning an ally of China is murdered, later that afternoon Beijing criticises the test, two days later the coal shipment is turned back. What's next?

When asked about the death of Kim Jong-nam, Chinese Foreign Ministry Spokesman Geng Shuang said his government had "seen the media reports" and that that they were "following the developments". I'll bet they are.

Image copyright Reuters Image caption Coal had been one of North Korea's main exports with most going to China

At a social function run by the Chinese military recently, I was speaking to a Chinese officer about the US demand that they do more to bring pressure on North Korea.

He shrugged his shoulders. He said they didn't know what the North Koreans would do next and that they had no idea what China could do to change their minds.

Yet by far and away the vast majority of trade in and out of the DPRK (Democratic People's Republic of Korea), as the country prefers to be called, is with China. If you take Chinese trade out of the equation there's not much left.

So why would Beijing put up with all this? Why put up with the waves of instability flowing out of Korean peninsula?

The answer is US troops.

Image copyright Reuters Image caption US and South Korean marines recently conducted a joint military exercise

It's often said that a meltdown in North Korea could lead to millions of refugees pouring into China but, even if this did happen, it would likely only be a temporary problem.

No. The real fear is that a complete collapse of the North Korean regime could lead to Korean unification, with American soldiers based in a country with a land border with China.

Beijing will not let that happen and Pyongyang's current ruler, Kim Jong-un, knows it.

So no matter how many times North Korea drives its powerful protector to distraction, in the end, Beijing believes it doesn't have much choice but to put up with its weirdness, with its basket-case economy, with its erratic behaviour and probably also with its pursuit of nuclear weapons.