Donald Trump has attacked Senator Richard Blumenthal of Connecticut in a series of tweets, calling the Democrat "phony Vietnam con artist" for saying a free press was an important part in finding out there was collusion between the Trump campaign and Russia.

Mr Trump made the comments as the President marks his 200th day in office, even as his campaign team is still under investigation by Congress, the FBI, and Special Prosecutor Robert Mueller for alleged ties to Russia and colluding on to interfere with the 2016 US election.

As has become his norm, the President went after Mr Blumenthal in a series of early morning tweets, referring to a 2010 controversy about the senator's claim to have served in the Vietnam War.

Mr Blumenthal had held a press conference apologising for his mistake, adding that he served as a reserve soldier during the war but not overseas, as he had originally implied.

Mr Trump had called for an investigation of Mr Blumenthal for the Vietnam service issue earlier this year, posting on Twitter: "When caught, he cried like a baby and begged for forgiveness... and now he is judge & jury."

The Connecticut senator was on CNN earlier speaking about his concerns about the Department of Justice's review of subpoena procedures for members of the press.

He said that it limited the media in doing its job. "That's a chilling effect on the press and on whistleblowers, and on information that doesn't involve classified data or disclosures, but in fact maybe just embarrassing or unwelcome," he said.

Without a "free press" the American people would not have known about the Trump campaign and transition team's meetings with Russian officials like former Ambassador to the US Sergei Kislyak, he added.