Seismic activity was detected Saturday near the site of North Korea's Punggye-ri nuclear site, but it is not known whether a nuclear test caused the development.
The first sign Pyongyang has conducted a nuclear test is usually seismic activity. North Korea has one nuclear test site -- Punggye-ri.
A magnitude 3.5 earthquake struck at 4:29 a.m. ET Saturday 22 kilometers (more than 13 miles) east-northeast of Sungjibaegam, North Korea, the US Geological Survey said. The depth of the earthquake was 5 kilometers (3.1 miles).
"This event occurred in the area of the previous North Korean nuclear tests," the USGS said. "We cannot conclusively confirm at this time the nature (natural or human-made) of the event."
The Comprehensive Nuclear Test Ban Treaty Organization, a watchdog group that works to end nuclear testing worldwide, said there were two seismic events Saturday in North Korea, neither of very large magnitude. The larger one had a 3.4 magnitude, the group's initial assessment found, which is much smaller than anything declared as a nuclear test in previous years.
The analysis indicated neither one was man-made, meaning they don't appear to be explosions. The locations are around 50 kilometers away (more than 30 miles) from the site of previous nuclear tests, the group said.
Two South Korean officials at the Korea Meteorological Administration told CNN that their analysis so far suggests Saturday's seismic activity around the nuclear site was not caused by an explosion or a collapse of the site.
"We assess that there is a slim possibility that it was caused by a collapse," said Park Jong-shin, one of the officials.
"Our analysis shows that it was a natural earthquake. However, we are carrying out a further analysis because there are concerns that it might have been a man-made earthquake."
A preliminary analysis shows the earthquake occurred 20 kilometers (12.4 miles) southeast of the North Korean nuclear test site, the official said.
China's earthquake administration said it was a magnitude 3.4 earthquake and cited a "suspected explosion."
War of words
Saturday's reports of seismic activity come at a time of rising tensions between the United States and North Korea, with North Korean dictator Kim Jong Un and President Donald Trump engaging in an escalating war of words over Pyongyang's nuclear program.
North Korea's foreign minister said this week that his country could test a powerful nuclear weapon over the Pacific Ocean in response to President Donald Trump's threats of military action.
Foreign Minister Ri Yong Ho spoke to reporters in New York during the UN General Assembly shortly after North Korean leader Kim Jong Un made an unprecedented televised statement, accusing Trump of being "mentally deranged."
In a rare direct statement, Kim said that Trump would "pay dearly" for the threats, and that North Korea "will consider with seriousness exercising of a corresponding, highest level of hard-line countermeasure in history."
"I am now thinking hard about what response he could have expected when he allowed such eccentric words to trip off his tongue," Kim said. "I will surely and definitely tame the mentally deranged US dotard with fire."
The forceful rhetoric from Pyongyang came after Trump threatened to "totally destroy" North Korea in a speech Tuesday to the General Assembly. Trump tweeted Friday that Kim was "obviously a madman" who would be "tested like never before."
North Korea's state-run Korean Central News Agency published articles Saturday with comments slamming Trump.
One -- titled "We will throw the irrational war fanatics of Trump's gang in the fire of justice" -- condemned Trump's UN speech as "rubbish."
"Trump, the president of so-called 'power,' is mad enough to wag his tongue without any consideration that he would 'totally destroy' a sovereign state."
The article said, "For more than seven decades since the foundation of the UN, no chief executives or diplomats including the preceding US presidents have openly called for the obliteration of another state" at the UN General Assembly.
"Trump's rubbish is the open declaration of war against our supreme dignity, state, social system and people, and an unpardonable extra-large provocation," the report said, adding that "the US warmongers will face unimaginable consequences."
"It is a foolish misjudgment if Trump's gang thinks that America is safe because it's far away from the Korean Peninsula."
"The fire roughnecks who enjoy playing with fire, will die in fire," the news agency said.
Concerns over nuclear program
To comply with a UN Security Council resolution calling for sanctions over North Korea's nuclear activity, China said Friday that it will limit its exports of refined petroleum products to Pyongyang as well as ban imports of the North's textiles.
The Chinese Ministry of Commerce said in a written statement that it will restrict exports of refined petroleum products from October 1, and ban the export of condensate oil and liquefied natural gas immediately.
A ban on textile imports from the North will go into effect immediately, the statement said.
Russian Foreign Minister Sergey Lavrov said this week that "we're witnessing a very dangerous confrontation spiral around (North Korea).
"We resolutely condemned the nuclear missile adventures of Pyongyang in violation of Security Council resolutions, but military hysteria is not just an impasse, it's a disaster," Lavrov said in a speech at the UN General Assembly.