US prosecutors on Monday accused Donald Trump's former campaign chairman Paul Manafort, who is facing multiple charges of money laundering, bank fraud and illegal lobbying, of attempting to tamper with witnesses.

A motion submitted by Special Counsel Robert Mueller, who is heading up the main investigation into alleged Russian election meddling, said that Manafort had contacted witnesses by phone and encrypted text messaging for the purpose of securing" materially false testimony."

According to the court document, he had sought to suborn perjury from witnesses who would be called to testify regarding the activities of the so-called "Hapsburg group" which allegedly carried out unlawful lobbying for Ukraine.

In February, the 69-year-old pleaded not guilty to charges of money laundering, illegal lobbying and lying, setting up the first trial to result from Mueller's investigation, due to begin on September 17. Then in March he pleaded not guilty to charges of bank and tax fraud, setting up a separate trial for July 10.

Mueller's investigation has now entered its second year, and continues to dominate US politics while menacing Trump's presidency.