Abe hopes to calm the political maelstrom with the appointment of Yuriko Koike, a former television newscaster who said she wanted to "boost confidence in the entire Abe administration."

"The security environment surrounding our nation remains serious, especially after North Korea's ballistic missile launches and its nuclear experiment," Koike said later in an inauguration speech to her ministry.

North Korea has angered Japan with its first nuclear test last October and with a number of missile test-firings over the past year.

Abe came in for fresh criticism for his handling of the crisis triggered by the remarks from Fumio Kyuma, who resigned as defense minister Tuesday after saying the 1945 nuclear attacks on Nagasaki and Hiroshima "couldn't be helped".

His replacement made clear that she did not condone his comments.

"Regarding the atomic bombing, Japan needs to play a leading role in nuclear disarmament and nuclear nonproliferation. Therefore in that sense we should not say (the nuclear attacks) 'couldn't be helped'," she told a press conference.

Kyuma's gaffe has reinforced perceptions that Abe, at 52 Japan's youngest post-World War II prime minister, cannot control the old guard of his party, which has ruled Japan almost continuously since 1955.

Kyuma "had become a huge political liability to Abe, who is already very unpopular and who didn't need this," said Professor Jeffrey Kingston, a specialist in Asian affairs at Temple University in Tokyo.

Abe "acted quickly this time, perhaps because he wants belatedly to demonstrate he has the leadership people think he doesn't have. But it's a little too late," he added.

The row has reopened emotional wounds in Japan, particularly among survivors of the devastating US nuclear attacks, which killed more than 200,000 people.

It comes as Abe struggles to persuade voters of the need for a more assertive foreign policy in the face of a nuclear armed North Korea, when many seem to be more concerned about bread and-butter issues such as pensions.