The head of the US military's Strategic Command is "assuming" North Korea's nuclear test earlier this month was of a hydrogen bomb, Defense News reported Thursday.
Pyongyang insisted the September 3 nuclear test was a hydrogen bomb but western powers have been reluctant to verify the claim.
The reported acknowledgement is the nearest the US has come to confirming it was an H-bomb, with a senior official previously saying only that there was nothing inconsistent with North Korea's claim.
"I saw the event, I saw the indications that came from that event," General John Hyten said. "I saw the size, I saw the reports and therefore, to me, I am assuming it was a hydrogen bomb."
Experts had said the blast had the hallmarks of a two-stage hydrogen bomb, which work on fusion, with a nuclear blast taking place first to create the intense temperatures required.
"The size of the weapon shows that there clearly was a secondary explosion," Hyten told reporters visiting the command with US Defense Secretary Jim Mattis, according to the outlet Defense News.
"To me that is the definition."
"You still haven't seen everything put together," he was quoted as saying. "But it's just a matter of when, not if."
The test, Pyongyang's sixth nuclear blast, prompted global condemnation and came after it carried out two intercontinental ballistic missile launches in July that appeared to bring much of the US into range.
Pyongyang said was the hydrogen bomb was small enough to fit onto a missile.
Hyten's remarks came as the US Pacific Command confirmed that Pyongyang had executed yet another test, firing an intermediate range ballistic missile over Japan and into the Pacific Ocean.
The launch comes after the Security Council slapped an eighth set of sanctions on North Korea, which vowed to accelerate its weapons program in response to the "evil" punitive measures.
Following that launch the United States Thursday called on China and Russia to take "direct actions" to rein in North Korea, which has ratched up global tensions with its rapid progress in weapons technology under leader Kim Jong-Un.
Tillerson called the latest raft of sanctions "the floor, not the ceiling, of the actions we should take".
President Donald Trump said last week that "military action would certainly be an option. Is it inevitable? Nothing's inevitable".
"It would be great if something else could be worked out."
Those comments followed his threat to rain "fire and fury" on Pyongyang if the North did not stop threatening the US and its allies.
In light of the latest launch the UN Security Council will hold a closed-door emergency meeting Friday at 3:00 pm (1900 GMT) at the request of the United States and Japan.