North Korea: 3.4-magnitude earthquake caused by suspected explosion at surface, China says

China's earthquake administration says a magnitude-3.4 earthquake has been detected in North Korea, which it suspects was caused by an explosion,

The report has raised fears that the isolated state has tested another nuclear bomb. The administration said in a statement on its website that the quake was recorded at a depth of zero kilometres.

China's official Xinhua news agency said the epicentre was in roughly the same place as a similar shallow earthquake on 3 September, which turned out to be caused by North Korea's sixth and largest nuclear test.

Yet the South's meteorological agency said it believed the quake was a natural event. 

An official at the agency said they were analysing the tremor, which they said was a magnitude 3.0, but the initial view was that it was a natural quake.

“We use several methods to tell whether earthquakes are natural or manmade,” said the official, who asked for anonymity.

“A key method is to look at the seismic waves or seismic acoustic waves and the latter can be detected in the case of a manmade earthquake. In this case we saw none. So as of now we are categorising this as a natural earthquake.”

All of North Korea's previous six nuclear tests registered as earthquakes of magnitude 4.3 or above. The last test on 3 September registered as a 6.3 magnitude quake.

There was no immediate reaction from China's Foreign Ministry.

Nuclear proliferation watchdog CTBTO was examining unusual seismic activity in North Korea, it said on Saturday.

“Analysts looking at unusual #seismic activity of a much smaller magnitude in the #DPRK,” CTBTO Executive Secretary Lassina Zerbo said in a Twitter post, adding more details were set to emerge later.

North Korea's Foreign Minister Ri Yong Ho, who is currently in New York for a United Nations meeting, warned on Thursday that Mr Kim could consider a hydrogen bomb test of an unprecedented scale over the Pacific.

Mr Ri told reporters in New York a test of "the most powerful detonation of an H-bomb" was one possible "highest-level" action against the US.

But he said that he did not know exactly what the North's leader was planning. "We have no idea about what actions could be taken as it will be ordered by leader Kim Jong-un," he added.

The quake comes amid heightened tensions around the Korean Peninsula after the North conducted a series of missile tests, firing two over Japan.

Donald Trump used a speech at the United Nations on Thursday to threaten to annihilate the isolated communist nation and derided the North's leader as "a little rocket man".

Mr Kim responded with an unprecedented personal statement in which he said Mr Trump would "pay dearly for his speech".