IT WAS just a matter of time until North Korea carried out another test of a nuclear bomb. Since Kim Jong Un inherited power from his father in 2011, he has accelerated the pace of trials of nuclear bombs and ballistic missiles. The imposition of new sanctions this year and stern talk at the ASEAN summit this week will not have pleased Pyongyang. So it was that on September 9th, the holiday that celebrates the founding of North Korea’s communist regime by Mr Kim’s grandfather, the country announced that it had carried out its fifth test. The force of the explosion appeared to be at least 10 kilotons, and perhaps as many as 30, making it by far the most powerful device North Korea has yet tested. It triggered an earthquake with a magnitude of 5.3, alerting South Korea of the event before its troublesome neighbour confirmed it. By comparison, North Korea’s previous test, in January this year, was estimated at six kilotons. (The prior three were in 2013, 2009 and 2006.)

North Korea’s increasing forcefulness is making the international community extremely nervous. It is thought to have a stockpile of some 20 devices to which it adds one every six weeks. The underground detonation carried out in January almost certainly was not the hydrogen bomb that North Korea claimed, but it has been followed by a series of missile tests. The government’s claim that it can now send a missile to America may be bluster, but it could almost certainly strike targets in both South Korea and Japan.

Most worrying is the question of whether North Korea can make a nuclear warhead small enough to put on one of those missiles, and robust enough to endure a trajectory that would take it into space and back. The North boasts that it can, although observers are sceptical. Yet there is no doubt that it is making rapid progress in its nuclear programme. It is clearly a priority for Mr Kim, who seems to be devoting even more of North Korea’s relatively meagre resources to it than his father did.

Japanese officials and those in other neighbouring states fret that the young Mr Kim is far less predictable than his father. Partly that is because the strength of his grip on the regime is unknown. That three of North Korea’s five nuclear tests have been carried out during his five-year rule suggests he wants to project strength domestically. That could be because he feels insecure, but might equally reflect self-confidence.

America, Japan and South Korea have responded with predictably harsh statements. Even China, North Korea’s closest thing to an ally, said it “resolutely” condemned the test. Barack Obama, America’s president, has made nuclear non-proliferation and disarmament something of a pet cause, pushing for a nuclear deal with Iran and visiting Hiroshima, one of the two Japanese cities on which America dropped nuclear bombs during the second world war. But there is worryingly little America and its allies can do to restrain Mr Kim.

The United Nations tightened sanctions on North Korea in March in response to the test in January. The new measures include a somewhat leaky ban on exports of coal and other minerals, one of North Korea’s main sources of foreign exchange. America added further sanctions of its own in July, specifically naming Mr Kim. Yet all this appears to be doing nothing to change Mr Kim’s behaviour for the better, and may simply be infuriating him.

America’s, Japan’s and South Korea’s main strategy is to exhort China to put more pressure on North Korea, since the North Korean regime relies on China for its economic survival. The Chinese government is increasingly frustrated by Mr Kim—it voted in favour of the UN sanctions this year, though it has not always applied them rigorously. But it worries that the collapse of Mr Kim’s regime might bring American troops to its frontier, along with a flood of refugees. And China’s relations with America and its allies in Asia are not at their best at the moment. It is disgruntled by South Korea’s agreement with America to host THAAD, a missile-defence system, and has been ruffling feathers in the South and East China Seas. The West’s best hope of restraining North Korea, in short, is a slender one.