SEOUL, South Korea — North Korea on Friday conducted its fifth atomic test, producing its biggest-ever explosive yield, South Korean officials said after monitors detected unusual seismic activity near the North’s northeastern nuclear test site.
South Korea’s Joint Chiefs of Staff said in a statement that ‘‘artificial seismic waves’’ from a quake measuring 5.0 were detected near the Punggye-ri test site.
The South’s Defense Ministry said it believed the North conducted a nuclear test, while European and U.S. monitoring services also detected similar seismic activity, with the U.S. Geological Survey calling it an ‘‘explosion’’ on its website.
A second North Korean nuclear test this year would raise serious worries in Washington because the North’s nuclear tests are part of a push for a nuclear-armed missile that could one day reach the U.S. mainland. A second nuclear test this year would be a defiant response to Western pressure on Pyongyang to halt its nuclear ambitions. The country has previously conducted tests every three to four years.
A South Korean Defense Ministry official, who refused to be named because of office rules, said that Seoul detected an estimated explosive yield of 10 kilotons and assessed that it was from a nuclear test.
After the fourth test, in January, South Korean lawmaker Lee Cheol Woo said Seoul’s National Intelligence Service told him that an estimated explosive yield of six kilotons was detected.
The 5.0 magnitude earthquake Friday is the largest of the four past quakes associated with North Korean nuclear tests, according to South Korea’s weather agency. Artificial seismic waves measuring 3.9 were reported after North Korea’s first nuclear test in 2006; 4.8 was reported from its fourth test this January.
North Korean leader Kim Jong Un has overseen a robust increase in the number and kinds of missiles tested this year. Not only has the range of the weapons successfully tested jumped significantly, but the country is working to perfect new platforms for launching them — submarines and mobile launchers.
The longer ranges and mobile launchers give the North greater ability to threaten the tens of thousands of U.S. troops stationed throughout Asia.
The seismic activity comes on the 68th anniversary of the founding of North Korea’s government and just days after world leaders gathered in China for the G-20 economic summit.
Any test will lead to a strong push for new, tougher sanctions at the United Nations and further worsen already abysmal relations between Pyongyang and its neighbors. North Korea is already one of the most heavily sanctioned places on earth, and many question whether the penalties work.
China, the North’s economic lifeline and only major ally, has previously offered cover to Pyongyang, though Beijing has expressed growing frustration with what outsiders call provocations.
North Korea is thought to have a handful of rudimentary nuclear bombs and has spent decades trying to perfect a multistage, long-range missile to eventually carry smaller versions of those bombs. After several failures, it put its first satellite into space with a long-range rocket launched in December 2012, and has since launched another such successful launch.
Experts say that ballistic missiles and rockets in satellite launches share similar bodies, engines and other technology. The U.N. calls the North’s long-range rocket launches banned tests of ballistic missile technology.
It’s unclear whether North has achieved the technology needed to manufacture a miniaturized nuclear warhead that could fit on a long-range missile capable of hitting the United States. Some analysts believe that the country has the ability to arm shorter range missile with warheads.
In January, North Korea claimed to have conducted a hydrogen bomb test, but many foreign governments and experts were highly skeptical about its claim. But whatever device the North detonated has likely put it a step closer toward its goal of developing a miniaturized warhead to be mounted on a long-range missile. Some analysts said the country needed only a couple of more test explosions before acquiring such a warhead.
North Korea’s persistent pursuit of missiles and nuclear weapons has long been one of the most intractable foreign policy problems for U.S. administrations. Another nuclear test would be the latest in a long series of events that test the current international approach to trying to contain the North’s nuclear ambitions.
On Tuesday, North Korea fired three medium-range Rodong missiles that traveled about 1,000 kilometers (620 miles) and landed near Japan. And last month, a missile from a North Korean submarine flew about 500 kilometers (310 miles), the longest distance achieved by the North for such a weapon. This worried many South Koreans because submarine-based missiles are harder to detect before launch than land-based weapons.
Diplomacy has so far failed to stop North Korea’s progress. Six-nation negotiations on dismantling North Korea’s nuclear program in exchange for aid were last held in late 2008 and fell apart in early 2009, when North Korea was led by Kim Jong Un’s father, Kim Jong Il, who died in late 2011.
North Korea blames the United States and South Korea for its nuclear program, saying long-running ‘‘hostility’’ from Seoul and Washington to its government makes the development crucial for the small country’s survival.
Among Pyongyang’s regular demands are for Washington to withdraw its troops from the region and to sign a peace treaty formally ending the Korean War.
The Korean Peninsula remains technically at war, as the 1950-53 conflict ended in an armistice. Washington stations more than 28,000 troops in South Korea as a buttress against any North Korean aggression. Tens of thousands more are in nearby Japan.