A definitive assessment is probably still weeks or even months away, but the size of the blast from North Korea's sixth and latest nuclear test suggests the weapon may be thermonuclear or use such fuel, analysts say.
The regime claimed it tested a hydrogen bomb on Sunday at the Punggye-ri test site. In photos released by the regime, leader Kim Jung Un posed for photos with a silver, peanut-shaped warhead meant for a ballistic missile. Real or not, the device resembled a two-stage thermonuclear design.
And while the device used in the test may not meet the official definition of a thermonuclear bomb, it may have employed thermonuclear fuel to enhance its power.
The yield for the North's latest nuclear test was estimated at between 100 kilotons and 300 kilotons -- by far its largest to date and many times the destructive power of the atomic weapons the U.S. dropped on Japan during World War II.
For comparison, the "Little Boy" atomic bomb at Hiroshima was estimated at between 13 kilotons to 18 kilotons, while "Fat Man" bomb at Nagasaki was estimated at between 54 kilotons and 75 kilotons.
This graphic from the U.S. Geological Survey's Earthquake Hazards Program shows a shake map from the magnitude 6.3 explosion-generated earthquake that took place Sept. 3, 2017, 22 kilometers east-northeast of Sungjibaegam, North Korea. A shake map is a product of the USGS Earthquake Hazards Program in conjunction with the regional seismic networks. Shake maps provide near-real-time maps of ground motion and shaking intensity following significant earthquakes. (U.S. Geological Survey graphic via Defense Department)
A hydrogen bomb relies on the heat generated from nuclear fission to trigger a fusion reaction and is far more powerful than an atomic bomb. For example, the U.S. in 1952 tested its first thermonuclear device, Ivy Mike, which generated a yield of 10.4 megatons, or 10,400 kilotons.
As for the current U.S. nuclear arsenal, the B61 bomb for U.S. fighters and bombers has a variable yield of up to 340 kilotons, the W78 warhead atop the LGM-30G Minuteman III intercontinental ballistic missile as much as 350 kilotons, and the W87 warhead on the Minuteman and the W88 warhead on the Trident II (D-5) submarine-launched ballistic missiles up to 475 kilotons.
A hydrogen bomb typically uses solid thermonuclear fuel, while a layered atomic bomb can employ thermonuclear gas or fuel, so the type and design of the North Korean bomb wasn't immediately clear. Also complicating the picture, the North prematurely claimed success with a thermonuclear weapon after a previous test in January 2016.
Even so, the latest exercise -- recorded as a 6.3 magnitude explosion about 13 miles east-northeast of Sungjibaegam by the U.S. Geological Survey's Earthquake Hazards Program -- shows North Korea is making advances in developing thermonuclear weapons, according to David Albright, president of the Institute for Science and International Security in Washington, D.C.
"The size of the seismic signal of the recent test suggests a significantly higher explosive yield than the fifth test," he told The New York Times. "Getting this high of a yield would likely require thermonuclear material in the device," he added, even if they haven't yet demonstrated the ability to outfit the device to a ballistic missile.