A small earthquake that hit North Korea on Saturday afternoon near the country's nuclear test site was most likely a natural tremor and not a nuclear explosion, some experts said.
A South Korea Meteorological Agency spokesman told Reuters that acoustic waves from a man-made explosion should have been detected.
"In this case, we saw none. So as of now we are categorizing this as a natural earthquake," he said.
China's Earthquake Administration said the quake was not a nuclear blast. It had said earlier that the 3.4 magnitude quake detected at 8:29 Universal time was a "suspected explosion."
The head of the Comprehensive Nuclear Test Ban Treaty Organization, Lassina Zerbo, posted on Twitter that the quake and a much smaller tremor Saturday at 4:43 Universal time were natural occurrences related to North Korea's most recent and largest nuclear test on September 3.
David Wright of the U.S.-based Union of Concerned Scientists agreed the latest quakes did not appear to have been caused by a new nuclear explosion.
"It seems likely that these small tremors are related to the shifts in the ground due to the recent large test," he said.
The U.S. Geological Survey said it could not confirm whether Saturday's seismic activity was man-made or natural.
The activity occurred only hours before North Korean Foreign Minister Ri Yong Ho was scheduled to address the U.N. General Assembly in New York. Ri warned on Thursday that North Korea might consider an even larger hydrogen bomb test in the Pacific region.
Ri's warning further escalated tensions around the Korean Peninsula that have been high since Pyongyang conducted its sixth nuclear test, prompting the U.N. to impose new sanctions on the country.
After Saturday's tremors, U.S. Air Force bombers escorted fighter jets in international airspace over waters east of North Korea. The Pentagon said the patrols were designed to display the numerous military options President Donald Trump has at his disposal to respond to the threat presented by North Korea's nuclear program.
Saturday's temblors occurred near the end of a week during which harsh rhetoric was exchanged between Trump and North Korean leader Kim Jong Un. Trump used his maiden speech at the U.N. to warn the U.S. would "totally destroy" North Korea if America or its allies were threatened. Kim responded by saying Trump was "mentally deranged."