Donald Trump's former personal lawyer Michael Cohen said that Mr Trump knew in advance about a June 2016 meeting in Trump Tower at which Russians offered to provide damaging information about his Democratic rival Hillary Clinton, according to reports.

CNN and NBC, citing unnamed sources with knowledge of the matter, said Cohen is willing to make that assertion to Special Counsel Robert Mueller, who is investigating possible collusion between the Trump campaign and Russia during the US 2016 presidential campaign.

Mr Trump again denied knowledge of the meeting in a string of tweets early on Friday morning. "I did NOT know of the meeting with my son, Don jr," he said.

Rudy Giuliani, Mr Trump's new lawyer said on Thursday Cohen "could not be believed". Referring to the Mueller investigation, he added: "If they rely on him ... it would destroy whatever case they have."

Federal prosecutors in New York are investigating Cohen for possible bank and tax fraud, and for possible campaign law violations linked to a $130,000 payment to adult film star Stormy Daniels, who has claimed she had a sexual encounter with Trump, and other matters related to Trump's campaign, a person familiar with the investigation has told Reuters.