President Donald Trump's eldest son and his former campaign chairman have been given approval by a U.S. special counsel to testify publicly to Congress as part of investigations into Russian meddling in the 2016 election, the top Democrat on the Senate Judiciary Committee said on Tuesday.

U.S. Sen. Dianne Feinstein told Reuters the special counsel, former FBI director Robert Mueller, said that Donald Trump Jr. and Paul Manafort, campaign manager from March to August, were free to speak to the committee.

Mueller is investigating allegations by U.S. intelligence agencies that Russia interfered to help Trump win the presidency and possible collusion between Moscow and the Republican's campaign. Russia denies meddling in the campaign, and Trump says there was no collusion.