Special Counsel Robert Mueller has added more charges against Paul Manafort, the ex-chairman of the Trump 2016 campaign, and indicted a top aide.
Russian citizen Konstantin Kilimnik - considered Mr Manafort's right-hand man in Ukraine - is accused of conspiring with Mr Manafort to obstruct justice.
Earlier this week Mr Manafort was accused of witness tampering.
Mr Manafort is also charged with money laundering, illegal foreign lobbying, and lying to federal officials.
The 69-year-old denies all charges against him.
The new charges against both men - known as a superseding indictment - alleges that they sought to tamper with witnesses ahead of Mr Manafort's trial next month.
The charges, which were filed in Washington DC on Friday, came days after Mr Mueller asked a judge to revoke Mr Manafort's $10m (£7.5m) bail, which has allowed him to remain under house arrest at his Virginia home, over the witness-tampering accusations.
Mr Kilimnik was a longtime employee of Mr Manafort's political-consulting firm, had done extensive lobbying work for him in Ukraine on behalf of the country's then pro-Russian president Viktor Yanukovych.
He has denied allegations that he has ties to Russian intelligence.
A clearer picture?
Analysis by Anthony Zurcher, BBC News, Washington
The legal pressure on Paul Manafort is growing - and now one of his top deputies is joining him in the prosecutorial crosshairs.
For the moment, Mr Manafort is one of the few targets of Mueller's Russia probe to push toward a criminal trial, rather than co-operate with the investigation. The special counsel's team may be hoping the prospect of pre-trial incarceration could help change Mr Manafort's mind.
Perhaps more important is that an alleged line from the Russian government to the Trump campaign could be getting clearer.
Konstintin Kilimnik, a Soviet-born former translator from Ukraine, is reported - by the New York Times and others - to be the "Person A" described in a previous Mueller court filings as having "ties to a Russian intelligence service" up to and including in 2016, when his boss, Mr Manafort, worked as the chair of Donald Trump's presidential campaign.
Mr Mueller has yet to assert that Mr Manafort's Ukraine and Russian connections resulted in any election-meddling. The skeleton for future allegations, however, can now be discerned.
Mr Mueller is investigating whether there was any collusion between the Trump campaign and Russia during the 2016 election, and whether the president unlawfully tried to obstruct the inquiry after the election.
Mr Mueller was appointed after US President Donald Trump fired FBI director James Comey last May.
On Tuesday he asked that Mr Manafort be detained in jail, claiming that he had been using encrypted communication to try to coach two potential witnesses' testimonies.
On Friday, Mr Trump denied that he was considering pardoning Mr Manafort.
"It's far too early to be thinking about it," Mr Trump told reporters before travelling to the G7 summit in Canada.
"They haven't been convicted of anything. There's nothing to pardon."