Accounts of the controversial payments surfaced this weekend when The New York Times reported that Manafort's name appears on a list of payments amounting to $12.7 million from 2007 to 2012.
A spokesman for the Ukrainian Anti-Corruption Bureau confirmed the details in the Times story.
The bureau spokesman emphasized that so far it has not been confirmed whether Manafort received the cash, since the bureau has not yet been able to confirm if signatures in the recipient column are Manafort's or not. The spokesman said the Anti-Corruption Bureau is conducting an analysis of the signatures.
“We can’t say 100 percent that this is his signature. We have an analysis going on to confirm whether this is his signature or not and, if not, who this signature belongs to," the spokesman said.
He confirmed that while the preliminary investigation is underway, the bureau is conducting "a criminal investigation on whole black accounts but not against individuals yet."
Manafort has denied receiving payments under the table from pro-Russian Ukrainian groups before becoming involved with the Trump campaign.
"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," Manafort said in a statement released to ABC News.
"Further, all of the political payments directed to me were for my entire political team: campaign staff (local and international), polling and research, election integrity and television advertising. The suggestion that I accepted cash payments is unfounded, silly and nonsensical," he said.
Manafort’s connections to Yanukovych and his party have surfaced intermittently over the past decade, first attracting notice during the 2008 U.S. presidential campaign when Manafort's partner in his firm, Davis Manafort Inc., was found to be advising John McCain at the same time as the firm appeared to be working to improve Yanukovych’s image abroad. As the Wall Street Journal reported then, Department of Justice filings showed that Manafort’s firm had hired a PR company in Washington to lobby on behalf of Yanukovych’s Party of Regions. The filings describe a firm owned by Manafort as being “directed by a foreign political party, the Ukraine Parties [sic] of Regions to consult on the political campaign in Ukraine.”
According to the filing, the PR firm, Daniel J. Edelman, Inc., was tasked with creating “a communications campaign to increase Prime Minister Yanukovych’s visibility in the U.S. and Europe” and to communicate “his actions toward making Ukraine a more democratic country.”
The WSJ reported again in 2010 just after Yanukovych won an expected return to the presidency, that members of the Party of Regions credited Manafort with the victory.
Manafort himself at the time refused to comment on this or other reports that he was advising the Ukrainian president.
Hillary Clinton's campaign manager, Robby Mook, released a statement saying that the new revelations marked "more troubling connections between Donald Trump's team and pro-Kremlin elements in Ukraine."
The Democratic National Committee on Monday called on Trump to "disclose any and all ties, financial or otherwise, that he or his campaign aides have to the Russian government.”
Allegations surrounding Manafort’s work in Ukraine are “especially troubling in light of the Russian government’s recently exposed cyber-attacks on the Democratic Party,” the committee said in a statement.