A US judge ruled Thursday that former president Donald Trump and his eldest children must testify under oath in New York's civil probe into alleged fraud at his family business.

The ruling is the latest legal blow to the 75-year-old as he fights numerous cases that threaten to complicate any bid for another run at the White House in 2024.

The Trumps have repeatedly tried to shut down the investigation by New York Attorney General Letitia James, who last month said she had uncovered "significant evidence" of fraudulent or misleading practices at the Trump Organization.

Following more than two hours of oral arguments, state judge Arthur Engoron rejected a plea by Trump, Donald Jr and Ivanka to quash subpoenas issued by James in December.

He ordered the trio to sit for depositions with James's office within 21 days. The Trumps are expected to appeal.

Their lawyers argued that the subpoenas in the civil case were an attempt by James to grab evidence for a parallel criminal investigation into the Trump Organization that she is involved with.

They argued that James was trying to bypass a New York state law that grants immunity to witnesses that appear before a grand jury in criminal cases.

Engoron said their argument "completely misses the mark," noting that neither the Manhattan district attorney, which is running the criminal investigation, nor James's office have ordered the Trumps to appear before a grand jury.

In his ruling, Engoron added that the Trumps could invoke their Fifth Amendment right not to incriminate themselves during questioning in the civil case.

He noted that Trump's son Eric had pleaded the fifth "more than 500 times" during a deposition for James's investigation in October 2020.

Engoron also rejected Trump's claim that the inquiry by James, a Democrat, is politically motivated.

Following the ruling, Trump's lawyer slammed James's investigation as "yet another politically motivated witch-hunt," one of the ex-leader's favorite refrains.

"The court clearly had its mind made up and had no interest in engaging in impartial discourse on this critically important issue," attorney Alina Habba said in a statement.

- 'Dereliction of duty' -

He ruled that the motive for the case was not "personal animus" but "sworn congressional testimony by (former Trump lawyer) Michael Cohen that the respondents were 'cooking the books.'"

He said that for James not to have investigated the allegations or subpoenaed the Trumps would have been "a blatant dereliction of duty."

James hailed the ruling, tweeting: "No one is above the law."

She said in January that her civil inquiry had found that the Trump Organization fraudulently overvalued multiple assets to secure loans and then undervalued them to minimize taxes.

If James finds evidence of financial misconduct, she can sue the Trump Organization for damages but cannot file criminal charges.

The Manhattan district attorney's probe into possible financial crimes and insurance fraud is very similar, however.

In that case, the Trump Organization and its long-serving finance chief, Allen Weisselberg, pleaded not guilty in a New York court to 15 felony fraud and tax evasion charges in July last year.

His trial is due to begin in the middle of this year.

At the heart of the twin investigations are a decade's worth of financial statements that Trump's longtime accountants Mazar's said last week were unreliable.

Mazar's announced it was ending its relationship with Trump in part because of James's findings.

Trump has so far kept Americans guessing about whether he intends to seek the Republican nomination again.

In Washington, he has been trying to prevent a congressional probe into the January 6 attack by his supporters on the US Capitol from accessing White House records related to that day.

In further bad news for the ex-president Thursday, a congressional committee asked a government agency to terminate the lease for one of his hotels in Washington before the Trump Organization can sell it.