Former US president Donald Trump said on Wednesday he refused to answer questions during an appearance before the New York state attorney general in a civil investigation into his family's business practices.

"I declined to answer the questions under the rights and privileges afforded to every citizen under the United States Constitution," Trump said in a statement.

The Constitution's Fifth Amendment offers protection against self-incrimination.

New York State Attorney General Letitia James is looking into whether the Trump Organisation inflated real estate values. Trump and two of his adult children, Donald Trump Jr and Ivanka Trump, fought to avoid testifying but lost.

James has said her investigation uncovered significant evidence that the Trump Organsation, which manages hotels, golf courses and other real estate, overstated asset values to obtain favourable loans and understated the values to get tax breaks.

Trump, a Republican, has denied wrongdoing and called the New York investigation politically motivated. James is a Democrat.

Trump in his statement said: "I once asked, "If you're innocent, why are you taking the Fifth Amendment?" Now I know the answer to that question. When your family, your company, and all the people in your orbit have become the targets of an unfounded, politically motivated Witch Hunt supported by lawyers, prosecutors, and the Fake News Media, you have no choice."

He added, "If there was any question in my mind, the raid of my home, Mar-a-Lago, on Monday by the FBI, just two days prior to this deposition, wiped out any uncertainty. I have absolutely no choice because the current administration and many prosecutors in this Country have lost all moral and ethical bounds of decency."

The FBI searched Trump's Florida estate on Monday, an escalation of the federal investigation into whether he illegally removed records from the White House as he was leaving office in January 2021.

A spokeswoman for James did not immediately respond to a request for comment, nor did a lawyer for Trump. (Reuters)