Is a North Korean bomb a direct threat to US security?

Not for the first time, I fear we're not paying enough attention to rising tensions in east Asia.

North Korea's underground nuclear test this week -- its third -- was a salutary reminder that all is not well in a region already facing a host of uncertainties.

Let's unpick just a few of them, starting with North Korea itself. It seems the world's last Stalinist dictatorship is now closer than ever before to having a nuclear bomb and a delivery mechanism which -- in theory -- could pose a direct risk to US security. (In other words, it can make a bomb small enough to be carried by a long-range missile all the way across the Pacific Ocean.)

No wonder President Obama responded to Pyongyang's latest example of nuclear sabre-rattling within hours of the test in his State of the Union address: "Provocations of the sort we saw last night will only isolate [North Korea] further, as we stand by our allies, strengthen our own missile defense, and lead the world in taking firm action in response to these threats."

Allies? Well, South Korea, obviously, but also Japan and Taiwan, both of which are growing increasingly twitchy at the dramatic changes in strategic power balance all around them. They know that they depend on the US security umbrella to enable them to sleep soundly at night -- and they need constant reassurance that the umbrella remains there for them.

Which brings us, as you thought it might, to China. A country that within the next five years or so will have overtaken the US as the world's biggest economy. A country with a military budget growing year by year, unlike the US military budget, which is being cut back.

A country that is arguing loudly with Japan over a group of uninhabited islands in the East China Sea -- a dispute that just last week reportedly came close to open military action when Japan claimed that a Chinese naval frigate locked its fire-control radar onto a Japanese ship near the islands.

When I wrote about this row last September, I suggested that it is more than a mere symbol of rival regional powers jostling for dominance. As I pointed out: "The islands are close to strategically important shipping lanes, and the waters around them offer rich fishing grounds and are thought to contain potentially lucrative oil deposits -- this isn't only about politics and pride by any means."

Japan calls them the Senkaku Islands and has controlled them since 1971, when they inherited them from the US, which had administered them since 1945. China calls them the Diaoyu Islands and says they've been part of China since as early as the 14th century and were ceded to Japan as part of Taiwan only after the first Sino-Japanese war.

Neither the new Chinese leader, Xi Jinping, nor the new Japanese prime minister Shinzo Abe can afford to be seen to be weak on the issue of the islands' sovereignty -- and with naval vessels from both sides playing a constant game of cat-and-mouse in the waters surrounding the islands, the risk of an "accident", whether provoked or otherwise, cannot be ignored.

And there's another complicating factor as well: China's relations with North Korea. Traditionally, Beijing is regarded as Pyongyang's one remaining ally -- Chinese trade and aid is all that keeps the North Korean Kim dynasty in place. Now, though, Beijing is mightily miffed at the latest North Korean nuclear test, which went ahead despite earnest -- and public -- pleas from Beijing to desist.

Being mightily miffed is one thing; but breaking with Pyongyang is something quite different. Beijing certainly doesn't like being snubbed, but nor does it want its unpredictable neighbour to go into melt-down following economic collapse and a political implosion. The end of North Korea would lead to the unification of the two halves of the Korean peninsula, and that would mean US troops, potentially, on China's border.

So, once again, all eyes are on China. The talk of tougher UN sanctions against North Korea seems to me to be utterly irrelevant -- as we've seen in countries as diverse as Iran and Cuba, all that sanctions tend to do is strengthen paranoid regimes and bring hardship to the people over whom they rule.

Perhaps the incoming South Korean president Park Geun-hye, who takes office in 10 days' time, will adopt a more nuanced approach to her northern neighbours; perhaps the new US secretary of state John Kerry will be able to come up with a joint approach together with Beijing.

There are a lot of new leaders in the region these days -- and that's both an opportunity and a danger. The oppportunity is for some new ideas to be tried out; but the danger is that political inexperience could lead to mistaken assumptions about what is feasible.