"Kim is very afraid to be killed like (Iraq's) Saddam Hussein, (Libya's) Moamer Kadhafi or Osama bin Laden," Satoshi Morimoto, who is now a national security expert, told a conference in Athens.
"So as long as they maintain nuclear capability they think they can survive," said Morimoto, a professor at Tokyo's Takushoku University who served as defence minister in 2012.
Morimoto argued that given available data, the weapon tested was likely a "small bomb with a launch missile."
He recalled a similar situation in 1953, when the Soviet Union said it had tested a hydrogen bomb, but the United States considered it a smaller-scale thermonuclear weapon.
Moscow's first 'true' hydrogen bomb test came two years later.
North Korea "is in the process of developing a hydrogen bomb, but not yet," Morimoto said.
