A former advisor to US President Donald Trump whose contacts with Russians set off the investigation into possible collusion with Moscow was jailed Friday for lying to the FBI.

US District Judge Randolph Moss sentenced foreign policy aide George Papadopoulos to 14 days in prison, acknowledging his guilty plea and his remorse, but noting that he "lied in an investigation that was important to national security."

Papadopoulos was the second person ordered to prison in the sprawling, 16-month Russia collusion investigation of Special Counsel Robert Mueller. It came just over two weeks after two former top Trump aides were convicted of felony crimes in cases that grew out of the probe.

Trump suggested the conviction was trivial for a probe that has cost millions since it launched in May 2017 -- while ignoring the 35 indictments, five guilty pleas and one trial conviction Mueller has racked up so far.

"14 days for $28 MILLION - $2 MILLION a day, No Collusion. A great day for America!" Trump tweeted.

Papadopoulos, 31, was an inexperienced London-based oil analyst when he joined the Trump campaign in March 2016 on the Republican candidate's national security advisory board.

Told the campaign's priority was to improve relations with Russia, within weeks Papadopoulos made contact with a mysterious professor, Joseph Mifsud, who touted links to the Kremlin.

Mifsud introduced him to others who ostensibly had connections to Russian President Vladimir Putin -- including a woman who claimed to be Putin's niece.

At a campaign meeting at the end of March 2016 Papadopoulos told Trump, then-senator and now Attorney General Jeff Sessions, and other campaign officials that he had connections in London that could set up a Trump-Putin meeting ahead of the November election.

Papadopoulos told CNN in a late Friday interview that Trump "gave me a sort of a nod" and "wasn't committed either way" at that meeting -- but that Sessions "was actually enthusiastic" about a Trump-Putin gathering.

He also said that Mifsud told him in April that the Russians had "thousands of emails" about Clinton -- which Papadopoulos described as a "momentous statement" but not a direct offer of assistance.

Weeks later, the stolen Clinton emails were posted online by what US intelligence chiefs now say were Russian intelligence actors.

The mysterious Mifsud has since vanished, CNN said.