SEOUL, South Korea — North Korea conducted its fifth underground nuclear test on Friday, its government said, despite threats of more sanctions from the United States and the United Nations. The latest test, according to South Korean officials, produced a more powerful explosive yield than the North’s previous detonations, indicating that the country was making progress in its efforts to build a functional nuclear warhead.
Later Friday, the North’s nuclear weapons institute issued a statement saying that the test was successfully conducted for a final check on the explosive power and other characteristics of a “nuclear warhead that has been standardized to be able to be mounted on” its ballistic missiles.
A statement from the South Korean military also said that an artificial tremor, registered as magnitude 5, had originated from Punggye-ri in northeastern North Korea, where the North has conducted its four previous underground nuclear tests.
A senior official at the Defense Ministry later told reporters that it had concluded that a nuclear detonation had caused the tremor.
The ministry estimated the explosive yield was equivalent to 10 kilotons of TNT, the most powerful detonation unleashed in a North Korean nuclear test so far, according to the official, who briefed reporters on the condition of anonymity. The South’s government estimated the North’s last nuclear test, conducted in January, at 4.8 magnitude with an explosive yield of six to nine kilotons. (By comparison, the atomic bomb dropped on Hiroshima in 1945 exploded with 15 kilotons of energy.)
North Korea’s first nuclear detonation, conducted in 2006, was largely dismissed as a fizzle, registering only as a 3.9 magnitude tremor with about one kiloton of energy. But its nuclear devices have steadily improved, producing bigger explosions with stronger seismic tremors in subsequent tests.
At the same time, the North has launched a series of ballistic missiles with growing ranges that it said were intended to carry nuclear warheads, though doubts persisted that the country had mastered the technology needed to produce a nuclear warhead small and sturdy enough to travel a long distance through Earth’s atmosphere.
Friday will be the 68th anniversary of the founding of the North Korean government. The country often celebrates its major holidays with displays of military might. Last week, it fired three ballistic missiles into the sea between the North and Japan, prompting the United Nations Security Council to urge the North to stop provocations or face more sanctions.
In March, the North Korean state news media reported that Mr. Kim had ordered that a “nuclear warhead explosion test” be conducted soon, as well as tests of ballistic missiles capable of delivering nuclear warheads. North Korea has since launched a series of ballistic missiles, including one fired from a submarine last month.
After the North’s nuclear test in January, and a long-range rocket launch weeks later, the United Nations Security Council imposed additional sanctions on the country, with the support of China. But the North has continued to flaunt its nuclear ambitions with a series of tests and claims about what it says are its growing technological capabilities.
Since inheriting power from his father, Kim Jong-il, in late 2011, Kim Jong-un has called for accelerating the North’s pursuit of long-range missiles and nuclear weapons in defiance of international pressure. Three of the North’s five nuclear tests have been conducted under his rule.
In recent months, North Korea has indicated that it is now capable of building a warhead compact and sophisticated enough to mount on an intercontinental ballistic missile. But such claims have been difficult to verify.
North Korea has never flight-tested a long-range missile, and officials and analysts in the region generally doubt that it has built a reliable ICBM. But the leaders of two government-run research institutes in Seoul have recently said that they believed North Korea was now able to mount a nuclear warhead on a short-range Scud or medium-range Rodong missile, if not on an ICBM.
Pyongyang said its Jan. 6 nuclear test was of a hydrogen bomb, which would have been a major escalation in its capacity for destruction.
Yet analysts were skeptical of the claim, saying that such a weapon would have generated a much bigger seismic wave. Some experts said the North might have tried to bolster the yield of a more basic device by using tritium, a radioactive form of hydrogen.