A minor but mysterious earthquake in North Korea on Saturday, close to where the country recently conducted a nuclear test, briefly set off concerns it might have been caused by an explosion, though South Korean officials tried to allay those fears.

The quake was detected in an area around Kilju, in northeastern North Korea, just six kilometres northwest of where the North conducted its sixth and most powerful nuclear test on Sept. 3, according to an official from South Korea's Korea Meteorological Administration.

The area isn't where natural earthquakes normally occur. Another South Korean expert said the quake could have been caused by geological stress created from the recent nuclear explosion.

"It could be a natural earthquake that really was man-made as the nuclear test would have transferred a lot of stress," said Hong Tae-kyung, a professor at the department of earth system sciences at Yonsei University in Seoul.

"The quake is small enough to suspect that it could have been caused by a tunnel collapse, and satellite data shows there have been many landslides in the area since the nuclear test."

China says explosion likely to blame

China's official Xinhua News Agency said the country's Earthquake Administration detected a magnitude 3.4 quake in North Korea and initially saw the likely cause as an explosion. The service said in a statement on its website that the quake was recorded at a depth of zero kilometres.

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Previous quakes from North Korea have indicated nuclear tests by the reclusive state, the most recent earlier this month. That quake was centred near North Korea's nuclear test site.

But the China Earthquake Administration later said Saturday's seismic activity was not from a nuclear explosion and had the characteristics of a natural tremor.

The South Korean weather agency official said the analysis of seismic waves and the lack of sound waves clearly showed that the quake wasn't caused by an artificial blast. She spoke on condition of anonymity, citing office rules.

U.S. agency unable to determine cause

The U.S. Geological Survey said that it detected a magnitude 3.5 quake in the area of previous North Korean nuclear tests, but that it was unable to confirm whether the event was natural.

North Korea's weakest nuclear test, its first one, conducted in 2006, generated a magnitude 4.3 quake. The USGS measured this month's nuclear test at magnitude 6.3. The latest test was followed by a second magnitude 4.1 quake that experts said could have been caused by landslides or a tunnel collapsing after the explosion.

Analysts examining satellite images of North Korea's mountainous test site after the latest nuclear test said they spotted landslides and surface disturbances that were more numerous and widespread than was seen from any of the five previous tests.

North Korea has been maintaining a torrid pace in nuclear and weapons tests as it accelerates its pursuit of nuclear weapons that could viably target the United States and its allies in Asia.

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With files from Reuters