The North Korean announcement was immediately condemned by Britain, China, France, Japan, the United States, the European Union and NATO, among others, even as experts cautioned that the North might have exaggerated its claims, as it did with its three previous nuclear tests, in 2006, 2009 and 2013.
For one thing, they said, the estimated yield, or energy, from the explosion appeared to be too small to be that of a hydrogen bomb. The North might instead have tested a so-called boosted-fission bomb - which involves placing a tiny amount of tritium, a radioactive form of hydrogen, at the core of an atomic bomb, but is more destructive than a traditional nuclear weapon.
Lee Cheol-woo, a member of the intelligence committee of the South Korean National Assembly, said his country's National Intelligence Service had estimated the explosive yield that 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 "hundred of kilotons or, even if it is a failed test, tens of kilotons," Lee told reporters.
Victor Cha, a Georgetown University scholar of North Korea and a former adviser to the National Security Council, said the North's test needed to be taken very seriously, even in the absence of proof that it was a hydrogen bomb. "We won't know for a while, if ever," he said. "From a national security perspective, I don't have the luxury of downplaying the North Koreans' claims, and would doubt the doubters."
Pyongyang's sole major ally, China, 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 Chinese Ministry of Foreign Affairs, said. "China is strongly against this act."
Russia joined the condemnation. "Such actions are fraught with further aggravation of the situation on the Korean Peninsula," Reuters quoted Maria Zakharova, a spokeswoman for Russia's Foreign Ministry, as saying.