US President Donald Trump's finance chief, a confidant who has worked for the family's real estate business since the early 1970s, has been granted immunity in the federal probe of former Trump lawyer Michael Cohen.

Key points: Allen Weisselberg received immunity to give grand jury testimony in the Cohen case

Allen Weisselberg received immunity to give grand jury testimony in the Cohen case Mr Weisselberg was mentioned on the Cohen tapes

Mr Weisselberg was mentioned on the Cohen tapes He is the one most likely to have comprehensive knowledge of Mr Trump's business deals

According to AP, the immunity granted to the Trump Organisation's chief financial officer, Allen Weisselberg, was restricted to his grand jury testimony last month in the Cohen case — specifically the allegations that Mr Cohen paid hush money to two women who claimed affairs with Mr Trump, according to one of the people, who spoke on condition of anonymity because they were not authorised to discuss the matter publicly.

The deal for Mr Weisselberg, which was first reported by the Wall Street Journal and NBC News, was seen as a major development since he was likely to have knowledge of every major personal and business deal Mr Trump has been involved in since his career as a real-estate mogul began.

Mr Cohen pleaded guilty on Tuesday to tax and campaign finance violations. And while not named in the Cohen case, Mr Weisselberg is believed to be one of two Trump executives mentioned in the suit who reimbursed Mr Cohen and falsely recorded the payments as legal expenses.

Mr Weisselberg's deal comes on the heels of several media reports that Mr Trump's longtime friend David Pecker, the CEO of National Enquirer publisher American Media Inc, has also been granted immunity in the Cohen probe, as well as the company's chief content officer, Dylan Howard.

The tabloid kept a safe containing documents about hush-money payments and damaging stories it killed as part of its cosy relationship with Mr Trump leading up to 2016 presidential election, AP reported.

Calls and emails to the Trump Organisation to reach Mr Weisselberg and general counsel Alan Garten were not immediately answered. An assistant said both were out of the office on Friday.

Sorry, this video has expired Ex-Trump lawyer Cohen pleads guilty in deal with prosecutors (Photo: AP/Richard Drew).

Weisselberg mentioned in Cohen tapes

Mr Weisselberg, an intensely private, loyal numbers-man for Mr Trump, was mentioned on an audiotape that Mr Cohen's lawyer released in July of Mr Cohen talking with Mr Trump about paying for Playboy model Karen McDougal's silence in the months leading up to the election.

Mr Cohen said on the tape that he had already spoken about the payment with Mr Weisselberg on "how to set the whole thing up".

In Mr Cohen's court appearance in Manhattan to enter his guilty plea on Tuesday, he did not specifically name the two women or even Mr Trump, recounting instead that he worked with an "unnamed candidate".

But the amounts and the dates all lined up with the payments made to Ms McDougal and porn star Stormy Daniels.

Mr Cohen said that the first payment was made in "coordination with and at the direction of a candidate for federal office" and the second payment was made "under direction of the same candidate".

He added that he arranged to make the payments "for [the] principal purpose of influencing [the] election".

All the amounts and the dates all lined up with the payments made to Ms McDougal and porn star Stormy Daniels. ( Reuters: Danny Moloshok )

The Trump Organisation eventually reimbursed Mr Cohen for the $US130,000 payment to Ms Daniels, accepting sham invoices and recording the money it sent to Mr Cohen as legal expenses. In court filings, prosecutors say two unnamed Trump Organisation employees — "executive one" and "executive two" — helped set up the reimbursement.

"Please pay from the Trust," executive one is quoted directing to another unnamed employee. "Post to legal expenses."

The "Trust" refers to the entity that Mr Trump set up after the election to hold his assets. He put the trust in the hands of his two two sons and Mr Weisselberg.

The identities of executive one and two are still unknown. Just because Mr Weisselberg and Mr Trump's sons were given control, that does not preclude others from handling the business.

Unlikely player in unfolding presidential drama

Mr Weisselberg is an unlikely player in the unfolding presidential drama, a low-profile employee who appeared in The Apprentice as a judge once but otherwise rarely drew the spotlight. He is not even mentioned in many of the biographies of his boss.

But as the perhaps the longest-serving employee in the Trump family business, he is rich repository of knowledge, and the idea of him answering questions to investigators under oath poses a new danger for the President as federal prosecutors in Washington and Manhattan dig deeper into Mr Trump's business affairs.

From his first job helping with the books for Mr Trump's father, Fred, in 1973, the Pace University graduate has gotten his fingers into nearly every aspect of the family business — vetting deals, arranging financing, auditing, managing cash — eventually rising to oversee all finances of its far-flung operations.

And aside from Mr Trump, he is perhaps best qualified to answer two of the big questions about the businessman-turned-president over the years: Is he really worth $US10 billion, as he claims, and what is in his tax returns? Mr Trump testified in a case years ago that Mr Weisselberg was the one who values his properties and other assets.

In addition to his title as chief financial officer, Mr Weisselberg holds executive positions at many Trump entities, including director of the Donald J Trump Foundation, which is being sued by the New York state attorney-general for allegedly tapping donations to settle legal disputes among other illegal uses. The White House has dismissed the suit as politically motivated.

Mr Weisselberg comes off in depositions in that case and others over the years as unobtrusive, loyal and undemanding.

Asked about what he thought of a last-minute order by Mr Trump to catch a flight to Iowa to tend to some business during the campaign, Mr Weisselberg said in one deposition that "it doesn't matter what I thought. He's my boss. I went".

He added that it was a rare trip for him: "I have never gone anywhere with Donald."

Sorry, this video has expired Donald Trump says 'hush money' did not come out of campaign funds.

AP