US investigators say they're checking possible links between Paul Manafort and alleged corruption by Ukraine's former president. Manafort was recently credited with helping Trump win the Republican nomination.
The probe is being carried out by the FBI and US Justice Department, broadcaster CNN reported late Friday, citing multiple US law enforcement officials.
The political strategist had advised Yanukovych for several years before joining the Republican hopeful's campaign team in March and becoming chairman in June.
The Associated Press reported Thursday that between 2012 and 2014 Manafort's firm allegedly orchestrated a covert Washington lobbying operation on behalf of Ukraine's then-ruling political party. Manafort and his deputy, Rick Gates, never disclosed their work as foreign agents as required under federal law, AP reported.
Questions raised over payments
Ukrainian anti-corruption authorities are investigating the political strategist after documents revealed possible secret payments allocated to Manafort to the tune of more than $12.7 million (11.21 million euros) from Ukraine's pro-Kremlin Party of Regions.
But the National Anti-Corruption Bureau emphasized that the mention of Manafort's name in the party's "black ledger" did not mean he had actually received the money.
Ukrainian investigators have said Yanukovych and his party engaged in widespread corruption. He fled to Russia following a popular uprising in 2014, which was followed by Russia's annexation of Ukraine's southern Crimea region and an insurgency by pro-Russian separatists in eastern Ukraine that Kyiv and many Western countries say has received considerable support from Moscow.
CNN said the FBI probe wasn't solely focused on Manafort, but he and another US political operative, Tony Podesta were named.
The US investigation is looking into the work of several firms linked to the former Ukrainian government, including the lobbying company Podesta Group and Mercury LLC.
The two firms have since hired lawyers to investigate whether they were misled into working on behalf of foreign governments or leaders.
Allegations strongly contested
Manafort has denied any wrongdoing. On Monday, in response to an earlier media report that carried the allegations, he issued a statement in which he said he had "never received a single 'off-the-books cash payment' as 'reported' by the 'The New York Times,' nor have I ever done work for the governments of Ukraine or Russia."
His resignation came two days after Trump shook up his campaign staff in the face of his sinking popularity ratings. In a previous shake-up, Manafort took over the running of the campaign from Corey Lewandowski, who was fired as campaign manager in June.
Democrat presidential hopeful Hillary Clinton's campaign team has seized on Manafort's departure, claiming it was another example of Trump's ties to Russian President Vladimir Putin.