North Korea's first test is largely believed to have fizzled, with a yield of less than 1 kiloton, and the second was between 2 and 7 kilotons.
"The first test almost failed. The second one showed they could basically do it. The third one showed that this is really working," said Won-Young Kim, a seismologist at Columbia University's Lamont-Doherty Earth Observatory.
The final intelligence task will be confirming or debunking North Korea's claim that this time around it tested a smaller, more advanced bomb. That is important because if the North is to field a nuclear weapon on the tip of a long-range missile, it must be lightweight. Making this determination will also depend on what materials leaked from the test, which experts can use to understand what kind of a device was detonated and infer how it was designed.
Experts have long been divided on whether North Korea has made much headway on clearing that hurdle, though the general consensus is they are not there yet. David Albright and Andrea Stricker, of the Institute for Science and International Security, said the latest test could be a measure of progress.
"Although more information is needed to make a sound assessment, this test could, as North Korea has stated, demonstrate this capability," they said in a statement. "ISIS has also assessed that North Korea still lacks the ability to deploy a warhead on an ICBM, although it shows progress at this effort."
Even so, they stressed North Korea could be years away from having a credible nuclear weapon that it could launch at the United States.
They said North Korea will need to conduct missile flight tests with a re-entry vehicle and mock warhead, increase the explosive yield of its warheads, possibly working to make them smaller, and improve the reliability of both its warheads and missiles.
Associated Press writers Foster Klug in Seoul, South Korea, and Kimberly Dozier in Washington contributed to this report.
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