US President Donald Trump says collusion is not a crime. Picture: AP
US President Donald Trump says collusion is not a crime. Picture: AP

Trump: ‘Collusion is not a crime’

US President Donald Trump has tweeted that "collusion is not a crime" as the first trial in federal special counsel Robert Mueller's Russia probe begins.

"Collusion is not a crime, but that doesn't matter because there was No Collusion (except by Crooked Hillary and the Democrats)!" Mr Trump wrote on Twitter.

The trial of his former campaign manager, Paul Manafort, on tax and bank fraud charges is set to begin in Virginia.

Mr Mueller is investigating whether Trump campaign officials worked with Moscow to try to sway the 2016 US presidential election.

While collusion is not a technical legal charge, Mr Mueller could bring conspiracy charges if he believes there was such work by campaign members with Russia.

Mr Trump's lawyer, former New York City mayor Rudy Giuliani did a round of interviews on Sunday, presenting what many believed to be a new legal defence - that collusion isn't a crime.

"You start analysing the crime, the hacking is the crime. The hacking is the crime," he concluded. "Well the president didn't hack. He didn't pay them for hacking."

Mr Trump has denied that the campaign worked with Russia to engineer his victory against Democratic candidate Hillary Clinton and bristles at the suggestion he might owe his White House victory to Moscow. Moscow has denied any meddling in the election.

Mr Manafort, 69, faces 18 criminal counts, which centre on allegations that he hid much of the $US60 million ($A81 million) he earned working for a pro-Russian politician in Ukraine in undisclosed overseas bank accounts and failed to pay taxes on it.

A Manafort conviction would give momentum to Mr Mueller, who has indicted or secured guilty pleas from 32 people and three companies since the probe started 14 months ago.