Washington (AFP) - President Donald Trump on Wednesday said he will not rule out a highly controversial pardon for his former election campaign chief snared in the Russia collusion investigation.

Paul Manafort has pleaded guilty to money laundering and other charges in connection with special counsel Robert Mueller's probe into whether the 2016 Trump election campaign colluded with Russian agents.

Trump told the New York Post that he has not discussed issuing a pardon to Manafort "but I wouldn't take it off the table. Why would I take it off the table?"

Manafort had been cooperating with Mueller in hopes of a lighter sentence, but the agreement has been torn up after Mueller declared that the former Trump confidant was lying.

Manafort now faces imminent sentencing. That has sparked speculation that Manafort may be expecting a presidential pardon -- something that would spark immediate accusations of White House meddling in the scandal and obstruction of justice.

Trump has always denied suggestions that he or his campaign got help from the Russians in his shock victory over Democrat Hillary Clinton.

But he has intensified his attacks on Mueller in recent days and on Wednesday he accused the prosecutor of pressuring witnesses to lie.

"This is our Joe McCarthy era!" Trump declared in a tweet, comparing the probe of his campaign to the notorious anti-communist campaign led by McCarthy in the Senate during the 1950s.

Trump claimed that "at least 3 major players are intimating that the Angry Mueller Gang of Dems is viciously telling witnesses to lie about facts & they will get relief."

Trump went on to retweet a crudely photoshopped image posted by a fan account called The Trump Train, under a banner headline that read: "NOW THAT RUSSIA COLLUSION IS A PROVEN LIE, WHEN DO THE TRIALS FOR TREASON BEGIN?"

Among the dozen figures it showed behind bars were former president Barack Obama, Bill and Hillary Clinton and Mueller.

The image also depicted Rod Rosenstein, Trump's own deputy justice chief, who had been overseeing Mueller's probe into Russia's election meddling until being supplanted earlier this month by a new acting attorney general seen as hostile to the special counsel's inquiry.

Story continues

Mueller has kept his investigation under close wraps, but signs are emerging of increasing action behind the scenes, leading to ever greater pressure on Trump.

The New York Times reported that Manafort's attorney has been briefing White House lawyers on his client's discussions with federal prosecutors, giving them valuable insights into Mueller's lines of inquiry.

Meanwhile, another witness, Jerome Corsi, said this week that Mueller's team was preparing to indict him for lying about an email he sent on August 2, 2016 alerting Trump campaign adviser Roger Stone to upcoming document dumps by WikiLeaks.

At key points during the campaign, WikiLeaks released email files hacked from the accounts of the Democratic National Committee and Hillary Clinton's campaign chief, John Podesta.