Donald Trump’s lawyers have sent a confidential letter to special counsel Robert Mueller saying the US president can ignore any demand that he testify about knowledge of Russian assistance to his 2016 campaign.

The 20-page memo also argued that there was nothing to discuss because Mr Trump could not possibly have obstructed justice in the Russia inquiry.

In what amounts to a veiled threat, the letter also warns Mr Mueller that the White House has the constitutional authority to “if he wished, terminate the inquiry, or even exercise his power to pardon”.

The Mueller’s investigation began in May last year after Trump fired FBI director James Comey, who at the time was examining Moscow’s influence on the ballot.

Trump has repeatedly rejected what he called “the nonsense of collusion with Russia” and recently called for Mueller to wrap up the probe.

In a tweet on Saturday, Trump accused the justice department and the special counsel of leaking his letters to the media and repeated his criticism of the Russia inquiry.

“When will this very expensive Witch Hunt Hoax ever end? So bad for our Country,” he tweeted.

-with AAP