At first glance, the valley of plum orchards and farmhouses behind Matsushiro village seems the sleepiest place imaginable. But if North Korea tests another nuclear bomb this is where the outside world will feel it first.

Behind the unassuming front door of a seismological observatory, 14 scientists maintain an unblinking vigil before a giant bank of screens. Tunnels stretch deep into the Nagano mountains, a network of secret bunkers originally designed to protect the Japanese Emperor during the Second World War.

Now they form the core of one of the world’s most sensitive earthquake detection centres and the nearest one to North Korea’s nuclear test site.

The alert level at the observatory — already described as tense by its director — has been raised again. As the rhetorical battle between North Korea and America grows more fierce, experts believe that the chance of a third test detonation by Pyongyang has grown more likely. “We are ready and we cannot miss it,” the observatory’s director, Naoya Mikami, told The Times. He acknowledges, however, that it is difficult to know for certain whether the blast his machinery detected on May 25 was nuclear or simply several thousand tonnes of conventional explosives. The screen he brings up for that date shows an enormous blast equivalent to a 5.3 magnitude quake, but not whether it was atomic.

The official line from Washington has been vague enough to allow speculation. The Office of the Director of National Intelligence in the US said that North Korea had “probably” conducted an underground test, and that it yielded “approximately a few kilotons” — much less than the 20 kilotons originally measured by Russian detection equipment.

The language of the North Korean regime reached new heights of bloodthirstiness this week when it warned of a “merciless” and “thousand-fold” military response if America or its allies made any provocative moves. The warning, issued via one of North Korea’s state newspapers, came hours after President Obama ended a summit meeting with his South Korean counterpart, Lee Myung Bak, in Washington. The two leaders vowed to work together to ensure that North Korea dismantled its nuclear weapons programme, with Mr Obama describing Kim Jong Il’s regime as a grave threat to the world.

Intelligence sources in Seoul have stepped up suggestions that Pyongyang is also about to test an intercontinental ballistic missile — possibly one with the range to hit the US. A Japanese report suggested that the missile might be aimed at Hawaii.

Media reports in South Korea and Japan have also suggested that North Korea may have moved a missile to a launching site in the northeastern region of Musudan-ri, though officials said that any apparent movement of goods and equipment to the area by rail was likely to be a decoy.

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