SEOUL, South Korea — The White House said Wednesday that initial data from its monitoring stations in Asia were "not consistent" with North Korea's claim that the nuclear test it carried out earlier in the day was its first test of a hydrogen bomb, a far more powerful weapon than the country had previously built.
The statement by Josh Earnest, the White House press secretary, came as the United Nations Security Council condemned the test after a two-hour closed-door meeting, and after China, Britain, France, Japan and other powers indicated that they would consider action against the country.
The United States did not indicate the basis for its skepticism. But the seismic wave left by the explosion was smaller than what most experts would expect from the detonation of a true thermonuclear weapon. Some experts said it was possible that the North had increased the yield of a more traditional device using tritium, a technique that has often been used in the 70-year history of nuclear weapons.
The true nature of the test may not be revealed until results are back from atmospheric testing, and even they may be inconclusive.
Earlier in the day, officials and analysts in South Korea cast doubt on the North's claim, saying that the seismological data from the test was more in keeping with a simpler uranium- or plutonium-based atomic device.
Lee Cheol-woo, a member of the intelligence committee of the South Korean National Assembly, said his country's National Intelligence Service had estimated that the explosive yield from the test was equivalent to 6 kilotons of TNT. (By comparison, the atomic bomb dropped on Hiroshima in 1945 exploded with 15 kilotons of energy.)
A hydrogen bomb would have yielded "hundreds of kilotons or, even if it is a failed test, tens of kilotons," Lee told reporters. The North's last nuclear test, in February 2013, set off a magnitude 4.9 tremor. The South estimated that the bomb detonated Wednesday resulted in a magnitude 4.8 seismic event, smaller than the 4.9 to 5.2 range that American, European and Chinese authorities had reported.
In Seoul, President Park Geun-hye convened an urgent meeting of her top national security aides. As South Korea's military increased its vigilance along the heavily militarized border with the North, its diplomats rushed to discuss with allies what Park called "strong sanctions" against Pyongyang.
She said that Pyongyang's claim, if true, "could potentially shake up the security landscape of Northeast Asia and fundamentally change the nature of the North Korean nuclear threat."
Prime Minister Shinzo Abe of Japan called the test "totally unacceptable" and "a grave threat to Japan's security," and he called on the Security Council to take "firm measures."
Pyongyang's sole major ally, China, has been increasingly impatient with the North's behavior and did not hide its displeasure Wednesday. "Today, despite the opposition of the international community, North Korea carried out a nuclear test," Hua Chunying, a spokeswoman for the Ministry of Foreign Affairs, said at a news conference in Beijing. "China is strongly against this act."
But so far, Beijing has not been willing to totally cut off Pyongyang. "China may strongly criticize the North, but once the issue arrives at the Security Council, it will focus on preventing sanctions that can affect the stability of the North Korean regime," said Chun Yung-woo, a senior adviser at the Asan Institute for Policy Studies and a former top negotiator for South Korea in nuclear talks with the North. "North Korea knows it too well."
China, Japan, Russia and the United States, along with the Koreas, are parties in the long-suspended six-nation talks aimed at ending the North's nuclear weapons program. At a summit meeting in Washington in October, Park and President Barack Obama urged Pyongyang to rejoin those negotiations and warned against a fourth nuclear test. But North Korea insisted that the United States first agree to negotiate a peace treaty with the North to replace the armistice that ended the Korean War in 1953.
The content and timing of the North's announcement came largely as a surprise, though Pyongyang's seemingly erratic behavior may be part of a calculated strategy to raise the stakes in any negotiations with the South and with the United States, and to bolster the reputation of North Korea's leader, Kim Jong Un, at home before an important party meeting.
Last month, Kim claimed for the first time that his country was ready to explode a hydrogen bomb, and in a New Year's Day speech, he called for "modernized" and "more diverse means of military strike." Still, the speech did not specifically mention nuclear weapons, and it also emphasized the need to improve living standards, so few if any officials or analysts had seen the test coming.
"This raises skepticism about our intelligence-gathering capabilities," said Kim Dong-yup, an analyst at the University of North Korean Studies in Seoul.
Since inheriting power after the death of his father, Kim Jong Il, in late 2011, Kim Jong Un has purged top members of his party and of the military elite — and he has proved to be more ambitious than his father in the pursuit of long-range missiles and nuclear weapons, even in the face of warnings from China.
Under Kim Jong Un, North Korea launched two long-range rockets, putting a satellite into orbit in the second attempt, in December 2012. The same year, the North revised its constitution to declare itself a nuclear power. Two months after the North's third — and Kim's first — nuclear test, in February 2013, his Workers' Party adopted a new national strategy: growing its nuclear arsenal and rebuilding its economy at the same time.
Kim wants his people to consider nuclear weapons the linchpin of their survival, but Washington and Seoul have repeatedly warned that the North's nuclear ambitions would only deepen its isolation.
"The benefits of being a nuclear power — to deter external threats and prove strength domestically — must in his mind outweigh the costs of facing yet another round of condemnation and sanctions, which Pyongyang is used to by now," said John Delury, a North Korea expert at Yonsei University in Seoul. "So with this test, he projects power and claims to enhance national security."
The Workers' Party is scheduled to hold its first full-fledged congress since 1980 in May. With no big improvements in the lives of North Koreans, Kim needs something else to show for his four-year-old rule.
"The biggest achievement Kim Jong Un can offer ahead of the party congress is his nuclear program," said Choi Kang, vice president of the Asan Institute. "It also means that things don't look good in the economic sector."
Cheong Seong-chang, a North Korea specialist at the Sejong Institute in South Korea, said the North had made its hydrogen bomb claim to position itself in the U.S. presidential campaign and, perhaps, to enter negotiations with the next administration with increased leverage.
In recent years, it has become increasingly clear from the North's pronouncements that its ultimate goal is to cement its status as a nuclear power, and to use that position to haggle with Washington and its allies to win diplomatic recognition and other concessions.
"There can neither be suspended nuclear development nor nuclear dismantlement on the part of the North unless the U.S. has rolled back its vicious hostile policy toward the former," North Korea said in a brief announcement Wednesday. It said Kim had made up his mind last month to conduct the hydrogen bomb test, and had signed a final order Sunday.
Chun, the former negotiator, warned that it would be a mistake for Washington and Seoul to rely on China using its economic leverage to force North Korea to change course. He said the North would budge only if the United States and its allies put in place sanctions strong enough to threaten its survival, like denying port calls for ships carrying North Korean cargo.
China, a permanent member of the Security Council, could veto any additional sanctions.
Choe Sang-Hun reported from Seoul and David E. Sanger from Washington. Javier C. Hernández contributed reporting from Beijing, Somini Sengupta from the United Nations and Jonathan Soble from Tokyo.