New York prosecutors are investigating financial dealings around some of Donald Trump's signature Manhattan real estate - including Trump Tower - as their criminal probe into the former president and his company nears completion.

People familiar with the matter told The Wall Street Journal that more than $US280 million ($A360 million) in loans taken out by the 74-year-old for Trump Tower; 40 Wall St; Trump International Hotel and Tower; and Trump Plaza are being examined.

District Attorney Cyrus Vance Jr's office is investigating the loans -- made to Mr Trump by subsidiaries of Ladder Capital Corp, a New York City-based real-estate investment trust - since 2012.

The years-old probe - led by Mr Vance, a Democrat - has been labelled a partisan "witch hunt" by the former president, while his lawyers have called it a "fishing expedition" aimed at gaining Mr Trump's tax returns.

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What began more than 12 months ago as an investigation into an alleged hush money payment to porn star Stormy Daniels has grown into a wider probe of what Mr Vance's office has described as "possibly extensive and protracted criminal conduct" at Mr Trump's business, including allegations of bank and insurance fraud.

Last week, as Mr Trump's historic second impeachment trial began, it was reported that the New York investigations were "accelerating", stepping up witness interviews in recent months and hiring forensic accountants.

Property records show that Mr Trump first turned to Ladder for a $US100 million ($A130 million) loan for his flagship Fifth Avenue tower in 2012 - with repayment due next year - and two years later, took a $US15 million ($A19 million) loan for Trump Plaza.

In 2015, Ladder lent him $US160 million ($A200 million) for his 71-storey 40 Wall St, followed by a $US7 million ($A9 million) mortgage for Trump International Hotel and Tower in 2016.

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The examination into Mr Trump's properties come as prosecutors continue their legal battle to secure eight years of the former leader's tax records, with a US Supreme Court decision expected soon on whether Mr Vance can obtain the returns.

According to white-collar defence lawyer and former Manhattan prosecutor, Daniel Horwitz, the documents could provide compelling evidence if they differ significantly from other financial statements by the Trump business.

Writing false information on a loan application with the intention of getting financial benefits one isn't entitled to can be a crime under New York law, according to legal experts.

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In September, Mr Trump's lawyers wrote that Mr Vance's battle to obtain the tax returns was resorting to "speculation and innuendo".

"This is all misdirection," they wrote. "(Vance) nowhere claims that his office is actually investigating any of the discredited, state, and recycled allegations of wrongdoing that are recounted in the press reporters he has compiled."

While Mr Trump was acquitted by the Senate on Sunday for "inciting an insurrection", his legal woes are just beginning across multiple states.

A separate civil-fraud investigation into properties including 40 Wall St and the Trump Organisation's 213-acre Seven Springs estate in Westchester is being conducted by New York Attorney-General Letitia James.

In Washington DC, Attorney-General Karl Racine is suing the Trump Organisation over alleged misuse of donors' funds during the 2017 inauguration, while in Georgia, prosecutors are reportedly investigating a phone call in which the self-described billionaire told election officials to "find" enough votes for him to win the state in last November's presidential election.

He's also staring down the barrel at 26 allegations of sexual misconduct and defamation.

During his presidency, Mr Trump's lawyers repeatedly invoked immunity and executive privilege to keep him from having to testify - but once again a private citizen, he's now far more vulnerable, having departed Washington last month in greater legal jeopardy than when he arrived.

"It's a potential avalanche," former federal prosecutor Kim Wehle told NPR in November.

"But this is, again, a man that is very used to using the legal system to his advantage."

Originally published as Criminal probe into Trump intensifies