A New York prosecutor seeking US President Donald Trump’s tax returns says he is investigating reported “protracted criminal conduct at the Trump Organization”.

Monday’s court filing suggests the inquiry is broader than alleged hush money payments made to two women who say they had affairs with Mr Trump.

The Supreme Court ruled last month that lawyers could examine the tax returns.

Mr Trump has repeatedly dismissed the probe, calling it a “witch hunt”.

Manhattan District Attorney Cyrus Vance Jr has filed a subpoena to obtain eight years of Mr Trump’s personal and corporate tax returns, which the Trump Organization is challenging.

The Republican president’s supporters accuse the Manhattan prosecutor, a Democrat, of pursuing a political vendetta.

Last week, Mr Trump’s lawyers filed a complaint arguing the subpoena was “wildly overbroad” and issued in bad faith.

Responding in court documents filed on Monday, lawyers for Mr Vance said that allegations of criminal activity at the Trump Organization date back “over a decade”.

His lawyers citied newspaper articles about supposed bank and insurance fraud at the Trump Organization and congressional testimony by the president’s former lawyer Michael Cohen, who said Mr Trump would devalue his assets when trying to reduce his taxes.

They said their inquiry extends beyond purported hush money payments made to adult film star Stormy Daniels and former Playboy model Karen McDougal in the lead-up to the 2016 election.

Such payments could violate campaign financing laws. The president denies the affairs took place.

Speaking to reporters, Mr Trump described the investigation as “Democratic stuff”.

“They failed with Mueller,” he said, referring to the justice department investigation by special counsel Robert Mueller that failed to establish the president had colluded with the Kremlin during his election campaign. “They failed with everything, they failed with Congress, they failed at every stage of the game.”

He added: “This is a continuation of the worst witch hunt in American history.”

Mr Trump, who inherited money from his father and went on to become a property developer, is the first president since Richard Nixon in the 1970s not to have made his tax returns public while running for office.

Media captionTrump: Supreme Court ruling a ‘witch hunt’ and ‘hoax’