A distinguished Chinese diplomat recently provided me with a fascinating insight into the nuances of Beijing’s relationship with its problematic neighbour in Pyongyang.

Prior to becoming ambassador in one of Europe’s major capitals, the diplomat had been given the challenging position of managing his country’s relations with North Korea.

And after spending a number of years handling the regime of North Korea's Kim Jong-un, the diplomat reached the conclusion that it was akin to having to deal with a delinquent teenager.

The recent actions undertaken by North Korea on behalf of  Kim have now gone well beyond the bounds of juvenile delinquency, with Pyongyang firing ballistic missiles over Japan and detonating an H-bomb said to be seven times stronger than the device America dropped on Hiroshima in 1945.

But the diplomat’s remark nevertheless helps to shed some light on our understanding of the strangely ambivalent approach Beijing tends to take whenever the issue of North Korea’s pursuit of nuclear weapons dominates the international agenda.

As North Korea’s main trading partner - China accounts for around 85 per cent of North Korean imports and exports - Beijing is about the only country in a position to exercise genuine influence over Pyongyang.

And there have been occasions when it has done so, most notably during the Six Party Talks process in 2009, whereby the US, China, Japan and South Korea agreed to provide significant quantities of food aid to North Korea. The Chinese have in the past supported a UN Security Council resolution condemning Pyongyang’s testing of a nuclear weapon in October 2006 when  Kim’s father, Kim Jong-il, was still in power.

But even though last weekend’s nuclear test is said to have caused tremors across the Chinese border, China’s more customary reticence about taking a harsh line with Pyongyang has once again set in.

There are many reasons that explain Beijing’s reluctance to join the international clamour for punitive action against Pyongyang over its recent bellicose action in the Asia-Pacific region, not least China’s interest in preserving some semblance of stability on the Korean peninsula.

Ever since Beijing gave its support to North Korea during the Korean War in the 1950s, it has seen its neighbour as an important bulwark against the democratic south, which is home to an estimated 29,000 US troops and marines. Chinese disquiet about Washington’s ultimate intentions in the Korean peninsula will not have been eased by the deployment of U.S. interceptor missiles in South Korea, ostensibly designed to protect the country from attack, but which Beijing regards as yet further evidence of American meddling in its backyard.

The leaders of China’s communist party are acutely aware that, if the regime in Pyongyang were to collapse, the most likely outcome would be the reunification of the Korean peninsula with Seoul as its capital, an outcome that could have grave consequences for the survival of China’s own one-party rule.

Even so, there will be a limit to how much Beijing is prepared to put up with the increasingly erratic behaviour of  Kim, whose seemingly endless gestures of defiance against the outside world are seriously undermining China’s pretensions to be taken seriously as a world power.

Last weekend’s detonation of what Pyongyang called a hydrogen bomb, for example, caused deep embarrassment for the Chinese leadership as it coincided with Beijing hosting a summit of the five-nation “Brics” club of emerging economies.

Chinese President Xi Jinping must now be worrying that  Kim is planning an equally provocative stunt during next month’s 19th Communist Party National Congress, which is being held in Beijing.

Not surprisingly, President Xi has a very low opinion of his North Korean counterpart, so much so that he has never even bothered to meet him. Like many aspirant Chinese of his generation, the Chinese leader views what China’s tourist authorities call “the hermit kingdom” with disdain.

Therefore, in weighing up how best to respond to the latest acts of provocation undertaken by  Kim, the US and its allies should take on board  Beijing’s equivocal attitude towards its difficult neighbour.

The Trump administration’s frustration with Beijing’s failure to rein in Pyongyang is perfectly understandable given that Donald Trump believed he had struck a deal with  Xi during their  Mar-a-Lago summit in Palm Beach, Florida, in April. The essence of the deal struck then was that Washington would delay pursuit of its protectionist economic agenda with regard to China in return for help on North Korea.

Beijing’s reluctance to fulfil its part of the deal, though, has not gone down well at the White House, where  Trump, who remains determined to address America’s enormous $347 billion trade deficit with China, has threatened a trade war with Beijing.

Any escalation in tensions between Washington and Beijing, though, are likely to be counter-productive, as the more  Trump tries to bully the Chinese to take action to resolve the crisis, the less likely the Chinese will be to oblige.

No parent is inclined to tolerate criticism of their offspring, no matter how badly-behaved they might be, and the same is true of China’s dysfunctional relationship with  Kim. The North Korean leader might be a deranged tyrant who is prepared to starve his people in pursuit of his nuclear ambitions, but he nevertheless remains an important ally of Beijing. And it is Beijing, not Washington, that ultimately has the influence to persuade  Kim to mend his delinquent ways.

The writer is a British journalist