A magnitude 3.5 quake was recorded in North Korea Saturday evening, and officials were trying to determine whether it was a natural tremor or a nuclear blast.
An official at South Korea's meteorological agency said they were analyzing the tremor, which they put at magnitude 3.0, but the initial view was that it was a natural quake.
President Trump and North Korean leader Kim Jong Un spent the past week trading insults and ratcheting up the already-high tension between the countries.
At the United Nations, North Korea's foreign minister, Ri Yong Ho, said Saturday that his country's nuclear force is "to all intents and purposes, a war deterrent for putting an end to nuclear threat of the USA and for preventing its military invasion, and our ultimate goal is to establish the balance of power with the U.S".
North Korea has refused to stop its missile and nuclear tests, despite successive rounds of United Nations sanctions.
On Thursday, Trump signed an executive order giving the Treasury Department more authority to cut off trade that helps finance North Korea's weapons and nuclear programs.
Ri Yong-ho described Mr Trump as a "mentally deranged person full of megalomania" on a "suicide mission".
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.
Saturday's quake follows an escalation of the combative rhetoric between the North Korean and U.S. leaders this week.
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"It could be a natural quake that really was man-made as the nuclear test would have transferred a lot of stress", he said.
China accounts for some 90 per cent of the North's trade, making its co-operation critical to any efforts to derail Pyongyang's development of nuclear weapons and long-range missiles. "S. dotard with fire".
A day earlier, Kim reacted to Trump's provocative speech at the U.N.by equating the USA president to "a frightened dog" and a "mentally deranged US dotard", employing an obscure insult for someone declining into senility.
"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".
He said Mr Trump's repeated insults about Mr Kim made "our rocket's visit to the entire United States mainland inevitable all the more". It said the operation showed the seriousness with which it took North Korea's "reckless behavior".
"It is a foolish misjudgment if Trump's gang thinks that America is safe because it's far away from the Korean Peninsula".
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.
And it's possible that North Korea is able to undermine them.
The US bombers' flight was the farthest north of the demilitarized zone separating North and South Korea that any US fighter jet or bomber has flown in the 21st century, the Pentagon said.