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N. Korea nuke yield estimate upgraded to 160 kilotons: Japan

Japan has raised its estimate of the explosive yield of North
Korea's sixth nuclear test to 160 kilotons, 10 times the force of the
atomic bomb dropped on Hiroshima in 1945, Defense Minister Itsunori
Onodera said Wednesday.

"This is vastly greater than previous North Korean nuclear
tests. We cannot rule out the possibility that this was a hydrogen
bomb test," Onodera told reporters.

"(North Korea) is evolving not just their ballistic missiles but
also their nuclear technology," he said.

Onodera said the new estimate is based on definitive seismic
data from a commission promoting ratification of the Comprehensive
Nuclear Test Ban Treaty.

The commission has informed Tokyo of its observation of
magnitude-6.1 shaking during Sunday's nuclear test, up from
provisional estimates of 5.8 and 6.0.

The government had initially put the yield at 70 kilotons, which
is still far greater than the yields estimated in North Korea's five
previous nuclear tests. It had later raised the estimate to some 120
kilotons.

The nuclear bomb dropped by the United States on the western
Japanese city of Hiroshima on Aug. 6, 1945, had a yield of 16
kilotons and one dropped on Nagasaki three days later came in at 21
kilotons. One kiloton has the explosive force of 1,000 tons of TNT.

North Korea said the test was a detonation of a hydrogen bomb
that can be mounted on an intercontinental ballistic missile,
prompting the U.N. Security Council to convene an emergency meeting
the following day.

While bombs like those dropped on Hiroshima and Nagasaki use
atomic fission to release energy, hydrogen bombs use an initial
fission reaction to force radioactive isotopes of hydrogen to fuse
together, giving off far more destructive force.

Earlier Wednesday, Onodera and U.S. Defense Secretary Jim Mattis
agreed in telephone talks to continue ramping up pressure on North
Korea in the wake of the nuclear test.

Speaking to reporters, Onodera said he told Mattis that Sunday's
test "was far greater (in scale) than previous nuclear tests and
presents a new, more grave and pressing threat to our country's
security." "Secretary Mattis expressed the same viewpoint," Onodera
said.

He quoted Mattis as saying the United States will defend Japan,
citing in particular the deterrence offered by the U.S. "nuclear
umbrella."

Onodera said Mattis also expressed his intention to actively
cooperate on the Japanese Self-Defense Forces' acquisition of the
land-based Aegis Ashore missile defense system.

In their roughly 20-minute conversation, Onodera and Mattis also
affirmed they will coordinate trilaterally with South Korea on the
matter.

They had made similar commitments in their last phone call on
Aug. 31, which followed North Korea's launch of a ballistic missile
across Japan into the Pacific Ocean.

After his call with Mattis, Onodera met Adm. Scott Swift, the
commander of the U.S. Pacific Fleet, at the Defense Ministry to
confirm their cooperation in responding to security issues.

Senior defense officials from Japan, the United States and South
Korea also held a videoconference on Wednesday to discuss their
response to North Korea. (Sept. 6)