A Ukrainian opposition politician who helped uncover off-the-books payments allegedly made to former Donald Trump campaign chairman Paul Manafort says he provided information on the matter to the FBI, but never received a follow-up.

Serhiy Leshchenko also told The Associated Press on Friday that his attempts to inform Ukrainian officials about the payments ran into roadblocks.

Manafort worked as a consultant to former Ukrainian President Viktor Yanukovych, who went on to be ousted in the wake of bloody street protests in 2014.

Leshchenko, a former investigative journalist, said that in February he "shared some documents" with the FBI about Manafort receiving a $750,000 payment via Kyrgyzstan.

He said: "Since that moment I have no feedback."

Leshchenko says letters he sent Ukraine's prosecutor-general about the payments were received only months later.