An accountant for US President Donald Trump's one-time campaign chairman Paul Manafort admitted in trial testimony that she helped falsify financial records at Mr Manafort's request to reduce his tax burden and help him qualify for loans.

Key points: Mr Manafort's accountant admitted tampering with documents in return for immunity

Mr Manafort's accountant admitted tampering with documents in return for immunity This is the first trial arising from the Mueller probe into Russian meddling

This is the first trial arising from the Mueller probe into Russian meddling Mr Manafort has plead not guilty to 18 different charges

Cynthia Laporta, who prepared Mr Manafort's tax returns starting in 2014, told a jury in federal court in Alexandria, Virginia, that she was testifying under an immunity agreement with the Government to avoid being prosecuted as Mr Manafort was charged with bank fraud and tax fraud.

One member of the jury nodded in apparent agreement when US District Judge TS Ellis cut off the prosecution's questioning to ask her if she was afraid of being prosecuted herself.

"Correct," answered Ms Laporta, explaining that she went along with accounting manoeuvrers suggested by Mr Manafort and his longtime business associate Rick Gates because she did not want to create problems for her firm or lose a top client.

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"I very much regret it," Ms Laporta said on the trial's fourth day as prosecutors build their case that Mr Manafort hid tens of millions of dollars he earned working for pro-Russian politicians in Ukraine to evade taxes.

Most damaging witness so far

Ms Laporta, the 14th witness to testify for the prosecution, was the most damaging yet for Mr Manafort in the first trial arising from Special Counsel Robert Mueller's probe of Russian meddling in the 2016 US election.

Mr Manafort has pleaded not guilty to 18 counts of bank and tax fraud and failing to disclose foreign bank accounts, charges that largely pre-date the five months Mr Manafort worked for Mr Trump, some of them as campaign chairman.

Once the jury had been dismissed for the day, Judge Ellis gave defence lawyers a green light for detailed cross examination of Ms Laporta, which will begin on Monday.

"You are not limited in your cross examination of her," Judge Ellis said.

A courtroom sketch depicts Paul Manafort (fourth from R) standing with his lawyers before opening statements. ( AP: Dana Verkouteren )

Both Ms Laporta and fellow accountant Philip Ayliff, her predecessor who handled Mr Manafort's tax filings at the firm KWC, testified that they had no knowledge that Manafort controlled foreign bank accounts.

The Government has provided trial evidence of Mr Manafort controlling a web of overseas accounts in Cyprus and elsewhere. Such accounts must be reported to tax authorities if they contain $US10,000 or more.

A conviction would give momentum to Mr Mueller's probe, in which 32 people and three companies have been indicted or pleaded guilty. Mr Trump, angered by any questions about the legitimacy of his election win, has called Mr Mueller's investigation a witch hunt and wants it to be shut down.

Manafort team to point the finger at Gates

After spending the first two days of the trial laying out Mr Manafort's lavish spending, the prosecution is now digging into how he accounted for the more than $81 million he made in Ukraine and his efforts to allegedly mislead banks to get loans once the income from Ukraine dropped off precipitously in 2014.

Mr Manafort's attorneys have signalled they will seek to blame Mr Gates, who was Mr Trump's deputy campaign chairman in 2016. Mr Gates pleaded guilty in February and is expected to testify against Mr Manafort, possibly next week.

President Trump has repeatedly called for the Mueller probe to be shutdown, insisting it is a witch hunt. ( AP: Evan Vucci )

Prosecutor Uzo Asonye focused some of his questioning on money transfers from a Cyprus-based company called Telmar Investments Ltd, which records showed had paid Mr Manafort's firm more than $6.7 million for consulting work.

That income posed a problem for MR Manafort when it came time to prepare his business tax returns in September 2015, Ms Laporta testified.

She said Mr Gates told her in a conference call the income level "was too high" and proposed reclassifying a portion of it as a loan.

Ms Laporta said she knew it was "inappropriate" but agreed to alter the records to show that Mr Manafort's firm received a $US1.2 million loan from Telmar in 2014, a change that would save Mr Manafort nearly a half million dollars in US taxes, Ms Laporta said.

Mr Manafort signed an agreement to account for that loan that was backdated, according to Ms Laporta and an exhibit shown to the jury.

Reuters