A Tokyo court on Friday approved a 10-day extension of jail time for Carlos Ghosn, the head of the Renault-Nissan alliance, as prosecutors assess allegations of financial misconduct.

The New York Times said Ghosn had been in detention since 19 November when he was arrested on his private jet soon after landing at Tokyo's Haneda Airport.

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The paper noted Ghosn has yet to be charged. But the Nissan board had voted unanimously to fire him as chairman while Mitsubishi Motors has also removed him as its chairman.

The report said Greg Kelly, Ghosn's former chief of staff and a Nissan board member, who was arrested at the same time, would also be detained for 10 more days. A former head of human resources and an American, Kelly has been described by the company as a "mastermind" of the alleged financial wrongdoing. Both men were in Tokyo for a board meeting.

The NYT noted, under Japanese law, prosecutors can detain suspects for up to 22 days without filing charges and can legally question them without a lawyer present.

The extended detentions gave Japanese authorities more time to interrogate the men, search Nissan's offices for evidence and determine whether to press charges. Prosecutors have said they suspect Ghosn earned around JPY10bn, or about $88 million, at Nissan from 2011 to 2015 but reported only half of it. The other half was to be parceled out as deferred compensation after Ghosn, 64, retired.

The NYT said Kelly was also alleged to have overseen the surreptitious purchase of millions of dollars' worth of housing for his boss through a Dutch company, Zi-A Capital, whose financial reports are opaque. Ghosn kept homes in Paris, Amsterdam, Rio de Janeiro and Beirut, Lebanon.

The report noted that, despite his accumulated wealth, Ghosn's outsize compensation packages at Renault and Nissan were a matter of contention at both companies, and each had sought to rein in his pay. Ghosn received around EUR8m, or about $9m, from Renault in 2017 and about the same amount at Nissan.

Ghosn had denied the allegations, according to Japanese public broadcaster NHK.

Kelly had also denied wrongdoing, according to his lawyer, Yoichi Kitamura.

Kitamura told the New York Times  Kelly maintained Ghosn had the right to set his own pay, and that the executive had decided to pay himself JPY1bn a year as chief executive. Ghosn and Kelly are accused of underreporting his compensation in required securities filings. Kelly said the securities report did not understate the pay, Kitamura said.

Kitamura said Kelly maintained he had consulted with an outside lawyer about a plan to defer Ghosn's compensation. According to Japanese media, Kelly had said he asked that lawyer to get approval from Japan's Financial Services Agency to ensure the plan was correct and that it did not need to be reported as current compensation.

Kitamura said Kelly believed the plan was legal under Japanese law after consulting with an outside lawyer.