WASHINGTON/UNITED NATIONS, Sept 23 (Reuters) - U.S. Air Force B-1B Lancer bombers escorted by fighter jets flew in international airspace over waters east of North Korea on Saturday in a show of force the Pentagon said demonstrated the range of military options available to President Donald Trump.
The patrols came after officials and experts said a small earthquake near North Korea's nuclear test site on Saturday was probably not man-made, easing fears Pyongyang had exploded another nuclear bomb just weeks after its last one.
The Pentagon said the mission was designed to show the many military options U.S. President Donald Trump had to deal with the grave threat presented by the North Korean nuclear program.
"This is the farthest north of the Demilitarized Zone (DMZ) any U.S. fighter or bomber aircraft have flown off North Korea's coast in the 21st century, underscoring the seriousness with which we take (North Korea's) reckless behavior," said Pentagon spokeswoman Dana White.
Saturday's seismic activity came just hours before North Korea's Foreign Minister Ri Yong Ho - who warned on Thursday that North Korea could consider a hydrogen bomb test of an unprecedented scale over the Pacific - was due to address the United Nations General Assembly in New York.
Ri did not respond when asked by reporters whether North Korea had conducted a new nuclear test.
Hours earlier seismologists around the world had detected a small earthquake near North Korea's nuclear test site
China's Earthquake Administration said the quake was not a nuclear explosion and had the characteristics of a natural tremor. The administration had said earlier the magnitude 3.4 quake detected at 0829 GMT was a "suspected explosion."
The CTBTO, or Comprehensive Test-Ban Treaty Organization, which monitors nuclear tests, and officials of the South Korean meteorological agency also said they believed it was a natural quake.
The Pentagon and the U.S. State Department did not immediately respond to requests for comment
A U.S. intelligence official and U.S.-based non-governmental experts said their initial assessment was that the quake was either natural or connected to North Korea's latest and largest nuclear test on Sept.3, and not caused by a new nuclear test.
"It seems likely that these small tremors are related to the shifts in the ground due to the recent large test," said David Wright of the Union of Concerned Scientists in the United States.
A U.S. government intelligence analyst said the events could have been a "mine-type" collapse of tunnels damaged by North Korea's previous nuclear test, but was more likely a small earthquake.
An official of South Korea's Meteorological Agency said acoustic waves should be detected in the event of a man-made earthquake.
"In this case we saw none. So as of now, we are categorizing this as a natural earthquake."
The earthquake, which South Korea's Meteorological Agency put at magnitude 3.0, was detected 49 km from Kilju in North Hamgyong Province, where North Korea's known Punggye-ri nuclear site is located, the official said.
All of North Korea's six nuclear tests registered as earthquakes of magnitude 4.3 or above. The last test registered as a 6.3 magnitude quake.
A secondary tremor detected after that test could have been caused by the collapse of a tunnel at the mountainous site, experts said at the time. Satellite photos of the area after the Sept. 3 quake showed numerous landslides apparently caused by the massive blast, which North Korea said was an advanced hydrogen bomb.
The head of the international nuclear test monitoring agency CTBTO said on Saturday that analysts were "looking at unusual seismic activity of a much smaller magnitude" than the Sept. 3 test in North Korea.
"Two #Seismic Events! 0829UTC & much smaller @ 0443UTC unlikely Man-made! Similar to "collapse" event 8.5 mins after DPRK6! Analysis ongoing," CTBTO Executive Secretary Lassina Zerbo said in a Twitter post, referring to the Sept. 3 test.
Russia's emergency ministry said background radiation in nearby Vladivostok was within the natural range.
The U.S. Geological Survey said it could not conclusively confirm whether the quake, which it measured at magnitude 3.5, was man-made or natural.
"The depth is poorly constrained and has been held to 5 km by the seismologist," USGS said.
Jeffrey Lewis, head of the East Asia Nonproliferation Program at the Middlebury Institute of Strategic Studies at Monterey, California, said: "Seismologists are very good at discriminating between earthquakes and explosions. I see no reason to doubt that it was an earthquake."
There was no immediate reaction from China's Foreign Ministry, but the news was widely reported by Chinese state media outlets and on social media.
Tensions have continued to rise around the Korean Peninsula since Pyongyang carried out its sixth nuclear test, prompting a new round of U.N. sanctions.
U.S. President Donald Trump called the North Korean leader a "madman" on Friday, a day after Kim dubbed him a "mentally deranged U.S. dotard" who would face the "highest level of hard-line countermeasure in history."
Kim was responding to a speech by Trump at the United Nations General Assembly in which Trump said the United States would "totally destroy" North Korea if it threatened the United States or its allies.
On Thursday Trump announced new U.S. sanctions that he said allows the targeting of companies and institutions that finance and facilitate trade with North Korea.
Earlier on Saturday, China said it will limit exports of refined petroleum products from Oct. 1 and ban exports of condensates and liquefied natural gas immediately to comply with the latest U.N. sanctions. It will also ban imports of textiles from North Korea.
North Korea's nuclear tests to date have all been underground, and experts say an atmospheric test, which would be the first since one by China in 1980, would be proof of the success of its weapons program.
North Korea has launched dozens of missiles this year, several of them flying over Japan, as it accelerates a weapons program aimed at enabling it to target the United States with a nuclear-tipped missile.
(Reporting by Andrew Galbraith in Shanghai, Ben Blanchard in Beijing, Christine Kim and Heekyong Yang in Seoul, Michael Shields in Zurich, Denis Pinchuk in Moscow, David Brunnstrom and Michelle Nichols at the United Nations and John Walcott and Idrees Ali in Washington; Editing by Lincoln Feast and James Dalgleish)