Japanese prosecutors are expected on Friday to request a 10-day extension to Carlos Ghosn's detention, as the former Nissan boss questioned whether he would receive a fair trial for alleged financial misconduct.

Courts in Japan very rarely refuse to grant such an extension, meaning the 65-year-old auto tycoon can expect to remain in his Tokyo detention centre under intense questioning until mid-April.

Authorities are looking into new allegations that Ghosn transferred some $15 million in Nissan funds between late 2015 and mid-2018 to a dealership in Oman.

They suspect around $5 million of these funds were siphoned off for Ghosn's use, including for the purchase of a luxury yacht and financing personal investments.

Prosecutors say Ghosn "betrayed" his duty not to cause losses to Nissan "in order to benefit himself."

Ghosn denies the allegations and says he is also innocent of the three formal charges he faces: two charges of deferring his salary and concealing that in official shareholders' document, and a further charge of seeking to shift investment losses to the firm.

Stephen Givens, an American attorney practising law in Japan since 1987, told AFP the latest allegations were the most serious yet.

"If the facts are true... that is stealing from the company, that is embezzlement, that is terrible," said Givens, who is not connected with the Ghosn case.

"If that's true, it is a serious charge. It differs very significantly from the previous charges that seem minor and technical. These are hurting Nissan or Nissan shareholders and benefiting Ghosn," Givens added.

Justice Minister Takashi Yamashita, a former Tokyo prosecutor himself, hit back at growing criticism of the Japanese system, sometimes described as "hostage justice" due to long detention periods for suspects aimed at forcing a confession.

"I understand that it is being handled appropriately in accordance with the stipulation of the code of criminal procedure. So the criticism is not warranted," Yamashita told journalists.

Ghosn launched a counter-attack in an interview with French television TF1 recorded just before his dawn arrest on Wednesday.

Describing himself as "a combative man and an innocent man", he vowed to "defend myself to the bitter end."

And he voiced concern that he would not be given a fair trial, with around 99 percent of trials in Japan resulting in a conviction.

"I have doubts over the way the judgement will take place. If there is a fair ruling, I am very confident but if it is not fair, I am worried about what will happen," said Ghosn.

At the end of the initial 10 days of questioning, prosecutors can request a further 10-day extension. After this, they must either press formal charges, release him without charge or re-arrest him to probe further allegations.

In his French TV interview, Ghosn lashed out at the conditions in the detention centre, saying he was deprived of his watch, forced to sleep with the light on and forbidden from contact with his loved ones.

"I wouldn't wish what I have suffered on my worst enemy," he said.

Ghosn spent 108 days in the detention centre in northern Tokyo before being dramatically released on bail of around $9 million on March 6, emerging from incarceration dressed in a workman's uniform and face mask in an apparent bid to avoid the media.

He had since lived in a court-appointed apartment in Tokyo without commenting on his situation despite huge international and Japanese media interest in his case that has shocked and surprised from the beginning.

However, just as reports began to surface that he could be rearrested, Ghosn emerged on Twitter to announce plans to hold a news conference on April 11.

With this now almost impossible, his lawyer Junichiro Hironaka said he had pre-recorded a video but refused to give details of the contents or when he would be released.