TOKYO - Japan's Defence Minister Tomomi Inada on Friday (July 28) announced her resignation to take responsibility for the cover-up of daily activity logs of Japanese troops in South Sudan.

Ms Inada, 58, explained that the buck of the scandal should stop with her, as defence chief, and volunteered to return one month of her salary. This was despite the fact that an internal Defence Ministry probe could not conclusively say if she had played any role in green-lighting the eventual decision to cover up.

Ms Inada told a press conference of her decision to resign: "I am keenly aware of the reponsibility as Defence Minister to lead the ministry and the Self-Defence Force (SDF).

"Following the cover-up allegations, there have been reports being leaked one after another, and this has caused serious damage to the trust by the public to the SDF and lower the morale of our troops."

Speaking to reporters minutes after Ms Inada's announcement, Prime Minister Shinzo Abe said he "regretfully" accepts Ms Inada's resignation and apologised to the Japanese public for the scandal. Ms Inada is the fourth minister to resign in the current Cabinet.

"As Prime Minister I am responsible for the appointment of my ministers, and I believe I must take to heart all the serious criticisms from the public. I would like to apologise very sincerely."

He added that Foreign Minister Fumio Kishida will take over the defence portfolio in the interim, with speculation rife that North Korea might be on the verge of firing another inter-continental ballistic missile (ICBM). Mr Abe said: "No vaccum should be created in Japan's national security."

Mr Kishida told reporters that he felt Ms Inada and the Defence Ministry have "striven to be firmly accountable to the public".

Chief Cabinet Secretary Yoshihide Suga, apologising to the public, also told a news conference that the government will "take all measures to prevent such a situation from reoccurring and will work hard to win back the public's trust".

When asked by reporters if he felt Ms Inada's resignation was belated, Mr Suga said: "Ms Inada has fully cooperated with the internal probe, and based on its findings decided to resign as the ministry's leader."

Japanese Ground Self-Defence Force (GSDF) troops had been deployed from 2012 to May this year in the young African country that is mired in civil war, as part of a United Nations peacekeeping mission. They were mainly involved in infrastructure projects.

But the situation in South Sudan deteriorated in July last year as heavy fighting erupted, killing at least 300 soldiers and displacing at least 36,000 civilians.

Japan, as a war-renouncing country with little public appetite for aggression, only allows troops to be sent abroad if there is a ceasefire in place. Defence experts suspect the logs contain incriminating details of Japanese troops being placed in dangerous situations in South Sudan, as the scandal erupted into a national controversy.

Their disclosure could have impeded Mr Abe's push for the SDF to take on expanded security responsibilities within the framework of the pacifist Constitution.

Besides Ms Inada, the Defence Ministry's top bureaucrat Tetsuro Kuroe will step down on Friday (July 28) and GSDF Chief of Staff General Toshiya Okabe, on August 8, media reports said.

According to the results of the internal probe, there were a series of legal violations involving Defence Ministry and military officials. Disciplinary measures were recommended for five officials, including Mr Kuroe and Gen Okabe.

But the probe report said it "could not rule out the possibility" that Ms Inada could have "overheard" remarks by her top aides as they planned to conceal the logs' existence.

A leaked memo that emerged earlier this week suggested that Ms Inada was not only well aware but also deeply involved in the eventual decision to cover up. Ms Inada has denied the veracity of the memo.

The report stressed investigators could not conclusively say if Ms Inada was asked to approve the decision to hide the logs from the public given contradictory testimonies to the fact.

The scandal began in December last year, when the Defence Ministry denied a media request for the information recorded in the logs by saying both hard and soft copies of the documents had been discarded.

It later uncovered electronic logs, but said these need not be released as they were recorded by individual soldiers and need not be taken as official.

Although the ministry later backtracked and released portions of the electronic logs, the controversy had already deepened given the flip-flopping.

Top SDF officials are said to have been aware of the logs' existence throughout.

A Cabinet reshuffle is widely expected next week, in which Mr Abe is expected to make wholesale changes in a bid to shore up his dwindling approval ratings that have fallen below 30 per cent in several media opinion polls. Ms Inada is not only the fourth to resign in the current Cabinet, but also the sixth minister to resign in Mr Abe's 4½ years in power.

The Nikkei Asian Review said Ms Inada's resignation might be too little, too late to quell a surge in discontent within the LDP, as some lawmakers believe that Mr Abe "exacerbated the problem by standing by (Ms Inada) until the very end".

Ms Inada, whom Mr Abe once regarded as his protege, has served the Liberal Democratic Party (LDP) as its policy chief for two years and was appointed in August last year.

But over the past year, her competence has come into question as she is seen to be unable to hold her own under relentless questioning in the Diet, while she has also drawn flak for a series of gaffes.

She was criticised for visiting the controversial Yasukuni Shrine, where Class A war criminals are enshrined, just a day after returning from a historic trip to Pearl Harbour with Mr Abe last December.

In April, the former lawyer came under fire for denying to the Diet that she had ever represented right-wing education operator Moritomo Gakuen - now under investigation for fraud - in court as a scandal erupted over an alleged sweetheart deal.

This was later found to be untrue, even if she had only momentarily stepped in for her lawyer husband, leading to opposition accusations of perjury.

Most recently, she also implied the apolitical defence apparatus supported the LDP candidate for the Tokyo assembly election earlier this month, in which more than half of the LDP's original 57 lawmakers in the 127-seat assembly were ousted.