The nuclear proliferation watchdog CTBTO has said that the two seismic events detected in North Korea on Saturday were probably not deliberate explosions.

Lassina Zerbo, CTBTO executive secretary, said in a Twitter post that the group detected two seismic events on Saturday. He added that they were still analysing the tremors, but they were "unlikely man-made".

"The events were in a zone that is not seismically active," Zerbo told Al Jazeera from the Austrian capital, Vienna.

He added that Saturday's tremors could be a delayed aftershock from North Korea's sixth and most powerful nuclear test on September 3.

"The sixth announced test by North Korea was big enough to create geological stress in the surrounding which could [weaken] any geological structure in the vicinity," Zerbo explained. "Any potential shock could be a consequence to that main one that happened on September 3 and that could take weeks."

Earlier on Saturday, China's earthquake administration said it had detected a magnitude 3.4 earthquake in North Korea, calling it a "suspected explosion".

The administration said in a statement on its website that the earthquake occurred around 08:30 GMT.

South Korea's weather agency said it was analysing the nature of the quake, but its initial view was that it was a natural earthquake.

South Korea's Yonhap news agency quoted a weather agency official who said that "a sound wave, which is usually generated in the event of an artificial earthquake, was not detected."

'Tense times'

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

"There was immediate concern because in the past North Korean nuclear tests have triggered artificial earthquakes," said Al Jazeera's Kathy Novak, reporting from the South Korean capital, Seoul.

The US Geological Survey (USGS) said it had detected a magnitude 3.5 earthquake in the area of the previous North Korean nuclear tests, but 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 tremor.

READ MORE: North Korea tensions: All the latest updates

According to USGS, this month's nuclear test generated a magnitude 6.3 earthquake.

Al Jazeera's Novak said Saturday's tremors have come at an "extremely tense time" in the Korean Peninsula.

North Korea's Foreign Minister Ri Yong-Ho threatened on Friday that his country could consider a hydrogen bomb test on an unprecedented scale on the Pacific Ocean.

His comments came after US President Donald Trump signed a new executive order allowing Washington to ramp up sanctions against North Korea.

Meanhwhile, North Korean leader Kim Jong-un and his US counterpart Trump have exchanged strong words for each other.

Trump called Kim a "madman" on Friday, a day after the North Korean leader dubbed him a "mentally deranged US dotard".