The campaign chairman for Republican presidential candidate Donald Trump has resigned.

Paul Manafort's resignation on August 19 comes amid a campaign shake-up by Trump and reports about Manafort's lobbying work for a Ukrainian political party and failure to register as a foreign agent.

Manafort was an adviser for the Ukrainian presidential campaign of Viktor Yanukovych and his pro-Russian Party of Regions from 2004 until 2010.

Trump's Democratic rival Hillary Clinton's campaign said the resignation does not end all questions about Trump's friendly relationship with Russia.

"You can get rid of Manafort, but that doesn't end the odd bromance Trump has with [Russian President Vladimir] Putin," Robby Mook, Clinton's campaign manager, said.

Ukraine's National Anticorruption Bureau said earlier this month that it found handwritten records showing some $12.7 million was designated to be paid to Manafort, though the bureau said it was unclear if Manafort received any of the money.

The resignation comes one day after the AP news agency reported that e-mails from Manafort's company show that he had lobbied in the United States for Ukrainian political figures, something that Manafort had denied doing.

U.S. law requires such foreign lobbyists to register as foreign agents, something that Manafort had not done.

Manafort has worked as an adviser to the presidential campaigns of several U.S. presidents and presidential candidates, including Gerald Ford, Ronald Reagan, and George Bush.

With reporting by donaldjtrump.com, AP, and dpa