Robert Mueller has referred three top Washington lobbyists to federal prosecutors over work they did for pro-Russian Ukrainian politicians.

They include Tony Podesta, whose brother John was campaign manager for Hillary Clinton's 2016 presidential run, and former Obama White House counsel Greg Craig.

Former Minnesota Republican Congressman Vin Weber was the third under investigation, with all referred in the past five months.

Tony Podesta, whose brother John was campaign manager for Hillary Clinton's 2016 presidential run, is among three lobbyists referred to prosecutors over work they did for pro-Russian Ukrainian politicians

The trio potentially didn't register their work for foreign agents, as required by law, and there were questions about how they were paid, according to CNN.

Failing to register such lobbying work carries a jail term of up to five years, but enforcing this has not previously been a priority.

Registering would have forced them to reveal the nature of their work in public reports to the Justice Department.

Mr Mueller, the special counsel investigating Russian interference in the 2016 election, handed off the cases to the Southern District of New York.

They were all uncovered during his investigation in connection to former Donald Trump campaign chairman Paul Manafort.

Former Obama White House counsel Greg Craig (left) was investigated for his work at law firm Skadden

Former Minnesota Republican Congressman Vin Weber was the third under investigation, with all referred in the past five months

The same lawyers prosecuting Manafort for financial fraud, in a trial that began on Tuesday, will investigate the cases.

Podesta's case concerns his work for his former company the Podesta Group, Craig for law firm Skadden, and Weber at Mercury Public Affairs.

No charges have been laid against any of them, or even may be later, but the cases were far enough along that Mr Mueller's team sent all three firms subpoenas for documents and interviewed Craig.

All three were recruited by Manafort to work for then-Ukrainian President Viktor Yanukovych, a close Moscow ally.

Podesta and Mercury were hired through the European Center for a Modern Ukraine, a nonprofit directed by Manafort, which prosecutors called a mouthpiece for pro-Russian Ukrainian politicians.

The firms were paid $1.1 million for the work and did not register with the Justice Department as required, according to the New York Times.

Robert Mueller (pictured), the special counsel investigating Russian interference in the 2016 election, handed off the cases to the Southern District of New York

They were all uncovered during his investigation in connection to former Donald Trump campaign chairman Paul Manafort, who is on trial for financial fraud

They instead making much less detail declarations to Congress and finally retroactively registered with Justice in 2017.

Skadden was hired directly by the Yanukovych Government to analyze its prosecution of former Prime Minister Yulia Tymoshenko.

The firm made a report in 2012, led by Craig, that attempted to improve the government's image to U.S. politicians.

This work was not registered despite Craig personally promoting the report to journalists and congressmen.

Podesta claimed it was misled by ECFMU into believing it was not connected to the Ukrainian Government and therefore its legal advice said it didn't have to register.

All three lobbyists were recruited by Manafort to work for then-Ukrainian President Viktor Yanukovych (pictured), a close Moscow ally

Mercury partner Michael McKeon said the firm welcomed any inquiry.

'We acted appropriately at every step of the process, including hiring the best lawyer in Washington we could and following his advice,' he said.

Both firms got Skadden to advise them on whether to register as working for foreign agents. The law firm claimed it too was misled by ECFMU.

Mr Mueller likely handed off the investigations to protect his investigation from being accused of going too far outside its remit.

It was not clear why the cases were referred to New York when the subjects were based in Washington, but it could claim jurisdiction as the payments were likely routed through there.