Donald Trump's ex-campaign manager Paul Manafort turns himself in to FBI over Russia meddling

Donald Trump's former campaign manager Paul Manafort handed himself into the FBI to face charges around alleged Russian meddling in the US presidential election, according to reports.

Paul Manafort was seen arriving at the Federal Bureau of Investigation Washington field office, television video showed, after being told to surrender to federal authorities, the New York Times and CNN reported.

The charges against Manafort would be the first arising from the investigation by Justice Department special counsel Robert Mueller, who was appointed to look into alleged Russian meddling to sway the election in favour of Trump.

Mueller's team is known to have interviewed several current and former White House staff as part of the inquiry and Manafort's business partner Rick Gates has also been told to surrender.

The charges against Manafort and Gates are not known but would include tax fraud, according to the Wall Street Journal.

A federal grand jury issued the indictment on Friday and a federal judge ordered it sealed, a source briefed on the matter told Reuters, adding it could be unsealed as soon as Monday.

Manafort, 68, was chair of Trump's 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.

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 Reuters.

Investigators also examined potential money laundering by Manafort and other possible financial crimes, according to the sources.

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 Trump's campaign chairman.

Trump has denied any allegations of collusion with the Russians and called the probe 'a witch hunt.' In a series of tweets on Sunday he dismissed the suggestions as a plot by the Democrats.

The Kremlin has also denied the allegations.

Just before the Manafort report came out, Trump senior adviser Kellyanne Conway insisted any charges would not necessarily implicate Trump or his campaign.

'Whatever happens today with the Mueller investigation, we don't even know that it has anything to do with the campaign ...,' Conway said on Fox.

The Russia investigation has cast a shadow over 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 Trump defeat Democrat Hillary Clinton by hacking and releasing embarrassing emails and disseminating propaganda via social media to discredit her.

Mueller is also investigating whether Trump campaign officials colluded with the Russian efforts.

Additional reporting by Reuters.