Donald Trump's campaign chief Paul Manafort has resigned after being demoted by Mr Trump in an attempt to deflect bad press over his aide's ethics and ties to the Kremlin.
With only three months until the US Election Day, Mr Manafort came under fire after The New York Times reported he received secret cash payments worth more than US$12 million (NZ$16.52 million) over five years from the political party of the Kremlin-backed former Ukrainian leader Viktor Yanukovich.
The allegations have further damaged the former Celebrity Apprentice host, who has been criticised for his pro-Russian foreign policy.
Democratic presidential candidate Hillary Clinton leads him by 8 percentage points among likely voters, according to the latest Reuters/Ipsos opinion poll.
Ukrainian MP Serhiy Leshchenko said the payments were made to Mr Manafort for finance services such as carrying out exit polls at elections, buying computers and conducting research.
Ukrainian anti-corruption investigators have corroborated the existence of what they called a "black ledger" of payments made by Mr Yanukovich's party, but stressed that they could not establish whether Mr Manafort himself received money.
"Manafort didn't work for free in Ukraine, he served the Party of Regions for over 10 years and it is clear that his work was paid," Mr Leshchenko told a news conference.
"The money was transferred in cash and it is impossible to trace the transactions, but I have no doubt as to the authenticity of these documents," he said.
"If Mr Manafort denies any allegations, I think he has to be interrogated into this case and prove his position that he was not involved," Mr Leshchenko added.
Mr Manafort has denied The New York Times' allegations, in a statement on Monday (local time).
"I have never received a single 'off-the-books cash payment' as falsely 'reported' by The New York Times, nor have I ever done work for the governments of Ukraine or Russia," he said.
Mr Manafort was brought on as Mr Trump's campaign chief in June to professionalise its approach, but struggled to get the wealthy businessman to rein in his freewheeling ways. The allegations of corruption appear to have been the final straw.
He's been replaced by Breitbart boss Steve Bannon as campaign CEO and Kellyanne Conway as campaign manager.