SEOUL—North Korea appeared to conduct a fifth nuclear test only months after its last nuclear detonation, highlighting the struggle of the U.S. to rein in the rising threat from dictator Kim Jong Un hours after President Barack Obama wrapped up a tour to Asia.
A magnitude 5.3 earthquake was detected near North Korea’s nuclear test site in the country’s northeast early Friday, the U.S. Geological Survey said, a reading that surpasses the magnitudes of tremors set off by the country’s previous nuclear tests. “Possible explosion, located near the location where North Korea has detonated nuclear explosions in the past,” the service said.
South Korea’s defense ministry said its initial analysis was that the explosion was caused by a nuclear device with a force equivalent to 10 kilotons of TNT.
Ned Price, a spokesman for the National Security Council, said the U.S. was “ monitoring and continuing to assess the situation in close coordination with our regional partners.”
The suspected explosion, on the anniversary of North Korea’s founding as a state, came a day after Mr. Obama said at a regional summit in Laos that he would continue to explore ways to reduce the threat from North Korea during his final four months in office. However, he acknowledged that making any progress on the issue would be difficult.
In January, North Korea detonated a nuclear device at the site, which it called its first test of a powerful hydrogen bomb. Outside experts suspect it was similar to three other less-powerful atomic bombs North Korea has tested since 2006. The January detonation triggered an earthquake of magnitude 5.1.
Since then, North Korean leader Kim Jong Un called for further nuclear and missile tests as Pyongyang elevated its threatening rhetoric and made a series of claims about being able to hit South Korea, Japan and U.S. bases in Asia with nuclear-tipped missiles.
Recent successful tests of missiles launched from a mobile carrier and a submarine show North Korea is gaining the capacity to threaten its enemies even if its military bases are destroyed.
Some experts say Pyongyang now appears to be only two or three years away from proving the ability to hit the continental U.S. with a nuclear-tipped missile.
South Korea has responded to North Korea’s missile progress by announcing a decision to deploy a U.S.-built advance missile defense shield by the end of next year.
“The real issue is whether North Korea can marry a nuclear device to a missile,” said Robert Kelly, a professor of political science at Pusan National University.
“The nuclear tests are unwanted of course, but not that destabilizing because we’ve known for a while that North Korea has functional nuclear weapons,” he said.
The earthquake occurred at 9.30 a.m. Seoul time (0030 GMT) about 230 miles northeast of Pyongyang, the USGS said. North Korean state media had no immediate comment on the quake.
In a statement on its website, the China Earthquake Networks Center, under the national seismology bureau, described the event as a suspected explosion.
Global reaction was swift.
“We are aware of seismic activity on the Korean Peninsula in the vicinity of a known North Korean nuclear test site,” Ned Price, spokesman for the U.S. National Security Council, said in a statement. “We are monitoring and continuing to assess the situation in close coordination with our regional partners.”
Japanese Prime Minister Shinzo Abe said his government was gathering information about a possible North Korean nuclear test.
“If this is a nuclear test, it is totally unacceptable,” Mr. Abe said. “We will protest strongly.”
Earlier this week, Mr. Obama called for China to work more closely with the U.S. to tackle North Korea’s threat. China, North Korea’s sole major ally and trading partner, says dialogue with Pyongyang is needed to deal with its nuclear advance.
Mr. Obama has said he would seek to tighten the sanctions against Pyongyang, and the United Nations General Assembly opens in New York later this month, providing world leaders who gather there each year with a forum to discuss the ongoing threat of North Korea’s nuclear program.
The U.N. gathering also will be the forum for a push by Mr. Obama to strengthen global nuclear non-proliferation measures. U.S. officials are considering pressing other countries for closer international adherence to a ban on nuclear tests.
Following North Korea’s nuclear test in January and a long-range rocket launch in February, the U.N. imposed stricter sanctions on Pyongyang designed to disrupt its international trade. The U.S. has also imposed further bilateral sanctions on North Korea and blacklisted Mr. Kim.
—Carol E. Lee contributed to this article.
Write to Alastair Gale at email@example.com