North Korea said it successfully tested a hydrogen bomb that could be fitted onto a long-range missile. The test, its first of this kind since September 2016, is in defiance of international sanctions and pressure from the United States, China, North Korea’s main political benefactor, and others. In that time, North Korea has also tested multiple intercontinental ballistic missiles (ICBMs) capable of reaching U.S. shores and medium-range missiles capable of striking its neighbors.

The test, a major escalation in the North’s recent belligerent actions, came at the end of a dramatic summer of threats lobbed back and forth between Pyongyang and Washington. Last month Japanese defense officials said North Korea had likely miniaturized a nuclear warhead that could be fitted onto an ICBM capable of reaching America. The revelation led to President Donald Trump declaring the U.S. would react with “fire and fury” if the North threatened it. On Sunday morning, he said the North’s “words and actions continue to be very hostile and dangerous to the United States,” and that it was a “rogue nation which has become a great threat and embarrassment to China, which is trying to help but with little success.”

Sunday’s test came hours after the North said it had developed a hydrogen bomb that could fitted onto an ICBM. What’s significant about that claim—and the subsequent test—is that a hydrogen bomb is several times more powerful than the nuclear devices the North has tested in the past. This one appears to have been more powerful than the bombs the United States dropped on Hiroshima and Nagasaki to end World War II.

The U.S. Geological Survey said it detected a tremor with a magnitude of 6.3 after the North’s test at 12:36 p.m., local time, at the Punggye-ri underground test site, in the northwest of the country. South Korea estimated the magnitude at 5.7—lower, but still “five to six times more powerful than” the North’s previous test in September 2016, said Lee Mi-Sun, the head of South Korea’s Meteorological Administration’s earthquake and volcano center. A second, weaker tremor, which came minutes after the first, likely indicated the “collapse” of tunnels at the test site, the USGS and South Korean officials said.

Notwithstanding North Korea’s claim that it tested a hydrogen bomb—which is far more powerful than the atomic bombs typically tested—it’s not clear if it was an actual hydrogen bomb that was detonated Sunday. The last time the North claimed to have detonated a hydrogen bomb was in January 2016, but many experts say that was a bomb “boosted” using tritium, a radioactive isotope of hydrogen that produces a higher yield during explosions. South Korean officials said the nuclear blast yield of Sunday’s test was between 50 and 60 kilotons, lower than the yield for a real hydrogen bomb, which can be in the range of 10,000 kilotons. This assessment would suggest that the bomb tested Sunday was not a true hydrogen bomb. But other estimates of the yield are higher.

Either way: What is known is the weapon is far more powerful than anything the Kim regime has previously tested, and that, combined with its regular ICBM tests with increasing range, makes the North a very threatening adversary. But perhaps still not an imminent one.

Last week, General Paul Selva, the vice chairman of the Joint Chiefs of Staff, told Bloomberg News that Kim still needs to overcome some technical hurdles before he can threaten the U.S. mainland. Here’s more:

First, North Korea would have to deploy a guidance and stability control system that could direct a long-range missile thousands of kilometers accurately without breaking apart, Selva said. Second, it needs a reentry vehicle housing the warhead that can survive the heat and stresses of an intercontinental ballistic launch. Third, it needs a nuclear weapon “that is small enough and stable enough to survive the trip,” he said.

Selva said the U.S. doesn’t know how fast Kim can achieve those capabilities, “but we’re certainly paying attention to determine what tools we have to either deter or slow his advance.”

Washington, while examining its strategic options, has also been working with China, South Korea, and Japan, to resolve the threat. Indeed, the North’s latest action comes after intense diplomatic pressure exerted on it by Beijing that has cut off large parts of its trade with Pyongyang in line with UN sanctions—though China, which views an unstable North Korea as detrimental to its own interests, says it lacks the ability to flip a switch and force North Korea to do its bidding.

Last week, South Korean President Moon Jae-in, who favors closer ties with the North, said: “I would consider that North Korea is crossing a red line if it launches an intercontinental ballistic missile again and weaponizes it by putting a nuclear warhead on top of the missile.” James Mattis, the U.S. defense secretary, said the U.S. was “never out of diplomatic options” with the North—suggesting the U.S. and its allies were looking for a peaceful way out of the crisis with the North. Kim Jong Un’s latest nuclear test Sunday makes that desire all that more difficult.