A 6.3 magnitude earthquake detected near a weapons test site in the country's northeast indicates a 100 kiloton explosion.
North Korea has conducted a sixth nuclear test, Japan has confirmed.
Japan's meteorological agency said the resulting tremors were at least 10 times as powerful as North Korea's previous nuclear test, last September.
Experts estimated that blast to have been around 10 kilotons - about five times larger than the bomb dropped on Nagasaki during World War II in August 1945.
Japan's foreign minister, Taro Kono, described the latest explosion as "extremely unforgivable".
The Tokyo government has registered a protest with the North Korean embassy in Beijing, he said.
South Korean news agency Yonhap said North Korea was due to make a "special announcement" at 7.30am UK time.
Earlier, the US Geological Survey said a magnitude 6.3 tremor struck the secretive state's northeast, near its weapons test site.
There are no immediate reports of damage or casualties.
Sky News' Asia Correspondent, Katie Stallard, said: "The significant point to take out of all of this is in terms of what response we see from the United States.
"We've seen over the last weeks and months increasing rhetoric from both sides, culminating memorably in Donald Trump's threat of "fire and fury" against Kim Jong Un's regime if they continued to make threats.
"So the question is given his previous rhetoric, his previous threats, how Donald Trump plans to respond to this."
Seoul based journalist Alex Jensen told Sky News: "We recently heard Donald Trump saying that all options are on the table… presumably they are still on the table.
"I think the US is going to be furious about this."
He continued: "It's ridiculous that Trump was saying only a few days ago that there was restraint in North Korea… when there was really no indication of the sort.
"I'm pretty sure you're going to see a lot of US hardware in this direction in the next few hours. Possibly it'll be a fly by and how is North Korea going to respond to that?"
State media quoted Mr Kim as saying it was a "thermonuclear weapon with super explosive power" and "all components of the H-bomb were 100% domestically made".
It claimed the hydrogen bomb's power is adjustable to hundreds of kilotons and can be detonated at high altitudes.
Experts are sceptical about the claim that Pyongyang has mastered hydrogen technology, but it is almost impossible to independently confirm statements about its highly secret weapons programme.
Melissa Hanham, of the Middlebury Institute for International Studies in California, said the images released by the North could not be proved real.
"We don't know if this thing is full of styrofoam, but yes, it is shaped like it has two devices," she said on Twitter.