Shanghai: A small earthquake near North Korea's nuclear test site on Saturday was probably not man-made, easing fears Pyongyang had exploded another nuclear bomb just weeks after its last one.

Chinese earthquake officials said the magnitude 3.4 quake detected at 5.29pm (AET) was a "suspected explosion", but both the CTBTO, which monitors nuclear tests, and a South Korean meteorological agency official said they believed it was a natural quake. The South Korean agency had logged the tremor as a magnitude 3.0 quake.

"We use several methods to tell whether earthquakes are natural or man-made," said the South Korean 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 man-made earthquake. In this case we saw none. So as of now, we are categorising this as a natural earthquake."