By Phil Stewart
OFFUTT AIR FORCE BASE, Neb. (Reuters) - The U.S. general who oversees America's nuclear forces said on Thursday he was making the assumption that North Korea did in fact test a hydrogen bomb on Sept. 3, crossing a key threshold in its weapons development efforts.
Although Pyongyang immediately claimed it successfully tested a hydrogen bomb, the United States had previously declined to characterize it.
Air Force General John Hyten, head of the U.S. military's Strategic Command, however, said he had a responsibility, as a military officer responsible for responding to the test, to assume that it was a hydrogen bomb, based on the size of the blast.
"I'm assuming it was a hydrogen bomb. I have to make that assumption as a military officer," Hyten told a small group of reporters who were accompanying Defense Secretary Jim Mattie on a trip to Hyten's headquarters in Nebraska.
"I'm not a nuclear scientist, so I can't tell you this is how it worked, this is what the bomb was. ... But I can tell you the size that we observed and saw tends to me to indicate that it was a hydrogen bomb and I have to figure out what the right response is with our allies as to that kind of event."
The North Korean nuclear test, its sixth and by far most powerful, prompted the U.N. Security Council to step up sanctions. It followed a series of North Korean missile tests, including one that flew over Japan and another that the U.S. assessed to be an inter-continental ballistic missile (ICBM).
South Korea's military said shortly after Hyten's remarks that North Korea fired an unidentified missile eastward from the Sunan district in its capital, Pyongyang.
A hydrogen bomb usually uses a primary atomic bomb to trigger a secondary, much larger explosion.
Such a weapon, with the first stage based on nuclear fission - splitting atoms - and the second on nuclear fusion, produces a blast that is much more power than traditional atomic bombs, or "pure fission" devices.
"The sheer destruction and damage that you can create with a weapon that size is significantly of a concern," Hyten said.
Hyten said that despite the nuclear and missile tests, North Korea still had not demonstrated that it had a reliable ICBM that could deliver a nuclear warhead.
But he noted it was only a matter of time before its scientists achieved that, given the pace of testing.
"It's just a matter of when, not if," he said, adding it could be months or years.
Experts doubt that President Donald Trump, like his predecessors, will be able to force North Korea to abandon its nuclear program through economic or diplomatic pressure.
Current and former U.S. officials have declined to comment on operational planning but acknowledge that no existing plan for a preemptive strike could promise to prevent a brutal counterattack by North Korea, which has thousands of artillery pieces and rockets trained on Seoul.
That raises the question of whether the United States might be able to live with a nuclear-armed threat from North Korea.
A senior Trump administration official, speaking to reporters last week on condition of anonymity, said it was unclear whether the Cold War-era deterrence model that Washington used with the Soviet Union could be applied to a rogue state like North Korea, adding: "I don't think the president wants to take that chance."
Hyten, who would command U.S. forces in a nuclear war, expressed confidence in the U.S. nuclear deterrent.
"Do we have the ability to deter North Korea from developing capabilities that could potentially threaten us? That's a different question," he said.
"But do I, U.S. Strategic Command, have the ability for the United States to deter an adversary from attacking the United States with nuclear weapons? Yes. Because they know the response is going to be the destruction of their entire nation."
(Additional reporting by Jack Kim and Christine Kim in Seoul, Reporting by Phil Stewart; Editing by David Gregorio and Cynthia Osterman)