North Korea's threat to test a hydrogen bomb over the Pacific Ocean "would be off the chart," former CIA Director Michael Hayden said Friday.
"We have no environmental concerns for safety and so on," Hayden, who also headed the NSA, told Jake Tapper on CNN. "That would be so dramatic."
North Korea's foreign minister issued the threat early Friday — telling reporters that any response to a U.S. attack on Pyongyang "could be the most powerful detonation of an H-bomb in the Pacific" — in the escalating rhetoric between dictator Kim Jong Un and President Donald Trump.
All six of North Korea's nuclear tests thus far, dating to 2006, have been conducted in underground tunnels.
No hydrogen bomb has been used in battle by any country, but more than 200,000 people died in Japan after the U.S. dropped the world's first atomic bomb on Hiroshima and then another three days later in Nagasaki in 1945 during World War II.
The bombings were so devastating that they forced Japan to surrender.
Experts say the most likely way North Korea would conduct an atmospheric test over the Pacific is to launch a long-range missile — probably flying over Japan — and have its nuclear warhead detonate in the skies over a remote part of the ocean.
Hayden, 72, a retired four-star Air Force general, told Tapper that any H-bomb testing by Pyongyang would be "such an affront to civilized life on the planet.
"It's been almost 35, 40 years since anyone has tested in the atmosphere — and that was done in a controlled environment within the national territory of China.
"So now you have this," Hayden said. "How would they do it?
"I think we might see signals if that was about to happen."
If so, the United States has "to bring unbearable pressure" on North Korea, Hayden said, adding that Trump would need to have "broader consensus" from other world leaders that such a test "can't happen."
He also discounted Trump's attacks on Kim's mental stability, telling Tapper that "I'll vote for sane, rational, calculating, within his context.
"And his context is personal and regime survival.
"He has to create the nuclear capacity that we find objectionable, the kind that reaches the United States," Hayden explained, "and then maybe he'll be willing to talk about a changed relationship."
The Associated Press contributed to this report.
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