A small earthquake near North Korea's nuclear test site was probably not man-made, the nuclear proliferation watchdog and a South Korean official say, easing fears Pyongyang had exploded another nuclear bomb just weeks after its last one.
On Saturday China's Earthquake Administration said the quake was not a nuclear explosion and had the characteristics of a natural tremor. The administration had said earlier the magnitude 3.4 quake detected was a "suspected explosion".
The CTBTO, or Comprehensive Test-Ban Treaty Organisation, which monitors nuclear tests, and officials of the South Korean meteorological agency said they believed it was a natural quake.
A South Korean meteorological agency official said they believed it was a natural quake.
"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," said the official, who asked for anonymity.
"In this case we saw none. So as of now, we are categorising this as a natural earthquake."
The Pentagon and the US State Department did not immediately respond to requests for comment
A US intelligence official and US-based non-governmental experts said their initial assessment was that the quake was either natural or connected to North Korea's latest and largest nuclear test on September 3, and not caused by a new nuclear test.
The seismic activity came just hours before North Korea's Foreign Minister Ri Yong Ho, who warned on Thursday that North Korea could consider a hydrogen bomb test of an unprecedented scale over the Pacific, was due to address the United Nations General Assembly in New York.
Ri did not respond when asked by reporters whether North Korea had conducted a new nuclear test.
A US government intelligence analyst said the events could have been a "mine-type" collapse of tunnels damaged by North Korea's previous nuclear test, but was more likely a small earthquake.
An official of South Korea's Meteorological Agency said acoustic waves should be detected in the event of a man-made earthquake but there were none.
All of North Korea's six nuclear tests registered as earthquakes of magnitude 4.3 or above. The last test registered as a 6.3 magnitude quake.
Russia's emergency ministry says background radiation in nearby Vladivostok was within the natural range.
The US Geological Survey said it could not conclusively confirm whether the quake, which it measured at magnitude 3.5, was man-made or natural.