WASHINGTON (AFP) - Fired former FBI director James Comey - who once led the government's probe into possible collusion between Donald Trump's campaign and Russia - certainly wasn't shy about voicing his opinion on the bombshell guilty plea of Michael Flynn.

And he used the Bible to make his point.

"'But let justice roll down like waters and righteousness like an ever-flowing stream' Amos 5:24," Comey wrote in a post on Instagram that included a picture of a flowing stream.

He also shared the verse on Twitter.

Trump abruptly fired Comey in May as he was leading a federal probe into possible collusion with a covert Russian effort to sway the 2016 US election in the Republican candidate's favor.

The probe was put in the hands of special counsel Robert Mueller, who on Friday unveiled a plea deal with former Trump national security adviser Michael Flynn in return for his cooperation with the investigation.

As part of the deal, Flynn pleaded guilty to lying to FBI investigators about his contacts with Russia's ambassador to Washington while former president Barack Obama was still in office.

Flynn said he reported on his contacts to Trump's presidential transition team, including to a "very senior official" who directed him to sound out the Russians on an anti-Israel resolution then before the UN Security Council, according to court filings.