Paul Manafort, the onetime chairman of Donald Trump's campaign, has been indicted by a federal grand jury on 12 counts including conspiracy against the United States, following an investigation into possible Russian meddling in the 2016 US election.

Key points: Charges against Paul Manafort include being an unregistered agent of a foreign principal

Charges against Paul Manafort include being an unregistered agent of a foreign principal Manafort and his associate Rick Gates have pleaded not guilty

Manafort and his associate Rick Gates have pleaded not guilty A third former campaign aide has pleaded guilty to giving the FBI false information

A third former campaign aide has pleaded guilty to giving the FBI false information Another hearing is set for Thursday — Manafort, Gates under house arrest

The charges were the first stemming from special counsel Robert Mueller's probe into possible ties between Mr Trump's presidential campaign and Russia.

The indictment filed in a federal court in Washington accused Mr Manafort, 68, and his associate Rick Gates, 45, of funnelling tens of millions of dollars in payments through foreign companies and bank accounts, most notably in Ukraine.

Mr Manafort and Mr Gates surrendered to federal authorities later on Monday (US time), and appeared in court overnight to face the charges brought by Mr Mueller's team — both men pleaded not guilty.

"The indictment contains 12 counts: conspiracy against the United States, conspiracy to launder money, unregistered agent of a foreign principal, false and misleading FARA statements, false statements, and seven counts of failure to file reports of foreign bank and financial accounts," the special counsel statement said. FARA is an acronym for Foreign Agents Registration Act.

"Between at least 2006 and 2015, Manafort and Gates acted as unregistered agents of the Government of Ukraine," the unsealed indictment said.

The judge ordered house arrest for both men and set a $US10 million ($13 million) unsecured bond for Manafort and a $6.5 million unsecured bond for Gates.

Sorry, this video has expired Mr Manafort (R) and associate Rick Gates (L) both pleaded not guilty to the charges. (Photo: AP/Reuters)

With unsecured bonds, they are released without having to pay but will owe money if they fail to appear in court.

Both men have reportedly had their passports confiscated and face at least 10 years in prison if convicted.

There will be another hearing on Thursday local time (5:00am AEDT on Friday).

Third aide pleaded guilty earlier this month

Meanwhile, in a separate development directly related to Mr Trump's 2016 election campaign, it emerged on Monday that former campaign aide George Papadopoulos pleaded guilty earlier this month to making false statements to FBI agents.

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The office of Mr Mueller said Mr Papadopoulos had told FBI agents he had been in contact with a professor in London who claimed to have information that would hurt Democratic presidential candidate Hillary Clinton in the form of "thousands of emails".

Mr Papadopoulos initially said such contacts occurred before he joined Mr Trump's campaign, but in fact he did not meet the professor until after he joined the campaign, the counsel's office said.

Neither Mr Trump nor his campaign were mentioned in the indictment and many of the charges, some of which go back more than a decade, have to do with Mr Manafort's work for Ukraine's pro-Russian government and political figures there.

"Today's announcement has nothing to do with the President, has nothing to do with the President's campaign or campaign activity," White House spokeswoman Sarah Huckabee Sanders said.

"The real collusion scandal has everything to do with the Clinton campaign.

"We've been saying from day one there's no evidence of Trump-Russia collusion, and nothing in the indictment today changes that at all."

The indictment said Mr Manafort and Mr Gates generated tens of millions of dollars of income from work for Ukrainian political parties and leaders and laundered money through US and foreign entities to hide payments.

The two concealed their work and revenue as agents of Ukrainian political parties, it said.

The charges against the men mark a sharp escalation in Mr Mueller's five-month-old investigation into alleged Russian efforts to tilt the election in Mr Trump's favour, and into potential collusion by Mr Trump's aides.

Sorry, this video has expired Sarah Huckabee Sanders distances Donald Trump from Robert Mueller's indictments

'There is no collusion!: Trump

Mr Trump responded on Twitter, saying the focus should be on Mrs Clinton.

Despite the indictment including 2017 activities, the US President said the charges related to events "years ago, before Manafort was part of the Trump campaign".

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"....Also, there is NO COLLUSION!" Mr Trump also tweeted.

Mr Manafort's attorney Kevin Downing later told reporters there was no evidence Mr Manafort colluded with the Russian Government.

"I think you all saw today that President Donald Trump was correct. There is no evidence that Mr Manafort or the Trump campaign colluded with the Russian Government," Mr Downing told reporters after Mr Manafort pleaded not guilty.

"Mr Manafort represented pro-European Union campaigns for the Ukrainians and … was seeking to further democracy and to help the Ukraine come closer to the United States and the EU.

"Those activities ended in 2014 over two years before Mr Manafort served in the Trump campaign."

Manafort, Gates, Papadopoulos: Who's next?

US intelligence agencies concluded in January that Russia interfered in the 2016 election. ( AP: Evan Vucci )

Manafort served the Trump campaign from June to August of 2016 before resigning amid reports he may have received millions in illegal payments from a pro-Russian political party in Ukraine.

Mr Mueller has been investigating Manafort's financial and real estate dealings and his prior work for that political group, the Party of Regions, which backed former Ukrainian leader Viktor Yanukovich, sources have told reporters.

Gates was a long-time business partner of Manafort and has ties to many of the same Russian and Ukrainian oligarchs.

He also served as deputy to Manafort during his brief tenure as Mr Trump's campaign chairman.

Mr Trump has denied any allegations of collusion with the Russians and called the probe "a witch hunt".

The Kremlin also has denied the allegations.

The Russia investigation has cast a shadow over Mr Trump's nine-month-old presidency and widened the partisan rift between Republicans and Democrats.

US intelligence agencies concluded in January that Russia interfered in the election to try to help Mr Trump defeat Democrat Hillary Clinton by hacking and releasing embarrassing emails and disseminating propaganda via social media to discredit her.

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Reuters/AP