KIM JONG UN, North Korea’s leader, has made no secret of his goal of developing a nuclear-tipped missile with enough range to destroy cities in America. What has confounded many experts is the speed at which his engineers are getting there. Only last year the consensus was that they would not succeed before 2020; now the target seems achievable within a year. If North Korea’s sixth nuclear test, on September 3rd, was of a two-stage nuclear device small enough to fit on a Hwasong-14 intercontinental ballistic missile (ICBM), as it claims, that marks a big advance over the previous nuclear test a year ago. The regime claimed that test, too, was of a two-stage device (also known as a thermonuclear or hydrogen bomb), in which an initial nuclear explosion is used to amplify a second one. But estimates of its yield (explosive power) varied between 10 and 30 kilotons, which even at the higher end is too little for a hydrogen bomb. David Albright, the president of the Institute for Science and International Security, a think-tank, believes that the North had instead tested a “boosted fission” device, in which lithium, deuterium and tritium are placed at the centre of a single-stage bomb, to increase its yield with a bit of fusion.

The most recent test was calculated by NORSAR, a Norwegian seismic agency, to have had a yield of about 120 kilotons, or about eight times that of the fifth test (see chart). That would be quite low for a two-stage thermonuclear device of the kind North Korea would like the outside world to believe it has developed, but not too powerful for it to have been a boosted fission device.

The day before the test, Mr Kim was pictured posing in front of a peanut-shaped container (almost certainly a plausible mock-up), with each end of the shell supposedly holding one of the device’s two stages. The official statement suggested that the test was of a device that could be adjusted to produce explosions that range between tens of kilotons and hundreds. That is a tricky feat to master, but a convenient way of explaining the relatively low yield for a genuine thermonuclear device.

Jeffrey Lewis of the Middlebury Institute of International Studies sees no reason to doubt North Korea’s claims. He argues that, based on the experience of other countries, six tests is more than enough to develop a thermonuclear device. Mr Albright remains more sceptical that the North has mastered two-stage technology. That matters, he says, because single-stage devices have a limited yield (even boosted-fission ones), whereas the power of a two-stage warhead is immense. With the latter, even with a relatively inaccurate guidance system, an entire city could be destroyed by a single missile. Either way, says Mark Fitzpatrick of the International Institute for Strategic Studies (IISS), the North Koreans will have learned a lot from the latest test and will probably have a true thermonuclear capability sooner rather than later.

If the story of North Korea’s path to a hydrogen warhead has been one of predictable, albeit accelerating, progress, the same cannot be said of the rapid advance it has made in long-range missiles over the past two years. Its intermediate-range (up to 4,000km) Musudan missile, which is also known as the Hwasong-10, had failed in six out of seven tests, the most recent in October last year. Yet since May, North Korea has twice successfully tested both the intermediate-range Hwasong-12 and the Hwasong-14.

Michael Elleman of the IISS says that no other country has advanced from a medium-range capability to an ICBM in such a short time. In a report published last month, he argues that the reason for the startling breakthrough was that North Korea had quite suddenly got hold of much better, more reliable rocket engines.

Mr Elleman believes that the Hwasong-12 and -14 are powered by versions of a high-performance Soviet missile engine known as the RD-250 which the North has acquired by stealth. Mr Elleman says that North Korea could not have developed and manufactured such large rocket motors, which generate 40 tonnes of thrust, from scratch. The only outfits with a history of producing this type of engine are Energomash in Russia and KB Yuzhnoye in Ukraine.

The latter has fallen on hard times, particularly since 2014, when Russian-backed separatists seized nearby parts of eastern Ukraine. Even before that, in 2011, North Korea had been caught trying to procure missile parts from KB Yuzhnoye. Mr Elleman reckons that, amid the chaos in the surrounding area, the North Koreans may now have succeeded, with the help of disgruntled employees or corruptible guards, in acquiring a “few dozen” RD-250 engines and transporting them home by train (they are only about two metres high and a metre wide).

Mr Lewis, among others, questions Mr Elleman’s theory and suggests that the North Koreans could have sought help from Iran, or that their own indigenous skills are being underestimated. The Bulletin of Atomic Scientists, however, agrees with Mr Elleman both about the type of engine and its probable source. Whoever is right, what is not in doubt is the sudden leap forward this year of North Korea’s missile programme.

The one piece of the jigsaw still needed to produce a credible nuclear-armed ICBM is a re-entry vehicle that will protect the warhead during its descent through the earth’s atmosphere. The re-entry vehicle attached to the second Hwasong-14 that was tested appeared to burn up in the terminal phase of its flight. But even if North Korea does not yet have one that can withstand the speed at which an ICBM travels, neither Mr Fitzpatrick nor Mr Elleman thinks building one will be an insuperable obstacle to engineers who have come this far this fast. North Korea’s nuclear-weapons programme is not quite there yet, but it is getting very close.