Washington: An organisation that was part of US President Donald Trump's transition team has claimed that special counsel Robert Mueller improperly obtained a trove of transition emails as part of the inquiry into Russian influence in the 2016 election.

The batch of emails totalling thousands of pages of communications was improperly provided to Mueller by the federal General Services Administration (GSA), the organisation claimed in a letter delivered to congressional investigators.

"This morning we sent a letter to Congress concerning the unauthorised sharing of private and transition emails with the Mueller team," lawyer Kory Langhofer said in an interview.

Details of the letter and the allegations from the transition organ known as Trump for America were first reported by Reuters.