An organisation established for US President Donald Trump's transition to the White House has said the special counsel investigating allegations of Russian meddling in the 2016 election had obtained tens of thousands of emails unlawfully.

Kory Langhofer, counsel to the transition team known as Trump for America, Inc. (TFA), wrote a letter to congressional committees to say Special Counsel Robert Mueller's team had improperly received the emails from the General Services Administration (GSA), a government agency.

Career staff members at the agency "unlawfully produced TFA's private materials, including privileged communications, to the Special Counsel's Office," according to the letter.

It said the materials included "tens of thousands of emails".

Mr Trump's transition team used facilities of the GSA, which helped manage US government bureaucracy in the period between Mr Trump’s November presidential election victory and his inauguration in January.

The Trump team's accusation adds to the growing friction between the president's supporters and Mr Mueller's office as it investigates whether Russia interfered in the election and if Mr Trump or anyone on his team colluded with Moscow.

Asked for comment, White House spokeswoman Sarah Sanders said: "We continue to cooperate fully with the special counsel and expect this process to wrap up soon."

The special counsel's office waved off the transition team's complaint.

"When we have obtained emails in the course of our ongoing criminal investigation, we have secured either the account owner's consent or appropriate criminal process," said Peter Carr, spokesman for the special counsel's office.

Democrats say there is a wide-ranging effort by the president's allies on Capitol Hill and in some media outlets to discredit Mr Mueller’s investigation.

Mr Trump himself has loudly declared Mr Mueller's effort as a waste of time.

"There is absolutely no collusion. That has been proven," the US President told reporters on Friday.

Russia denies interfering in the election.

On Friday, Representative Adam Schiff, the top Democrat on the House of Representatives Intelligence Committee, said he feared the committee's Republican majority intends to close its investigation of the topic prematurely.

Some Republicans have argued that Mr Mueller is biased against the president and should be fired.

Mr Langhofer's letter was sent to the US Senate Committee on Homeland Security and Government Affairs, and the US House Committee on Oversight and Government Reform.

It asked for Congress to act immediately "to protect future presidential transitions from having their private records misappropriated by government agencies, particularly in the context of sensitive investigations intersecting with political motives."

The letter said Robert Mueller's office obtained the emails despite the fact that it was aware the GSA did not own or control the records.

It said the special counsel's office has "extensively used the materials in question, including portions that are susceptible to claims of privilege" without notifying the Trump for America team.

On the transition team were a number of aides who were later caught up in Mr Mueller's investigation, such as former national security adviser Michael Flynn.

Mr Flynn pleaded guilty this month to lying to the FBI about his contacts with Russia.

Mr Langhofer, the Trump transition team lawyer, wrote in his letter that the GSA's transfer of materials was discovered on 12 December and 13 December.

The FBI had requested the materials from GSA staff on 23 August, asking for copies of the emails, laptops, mobile phones and other materials associated with nine members of the Trump transition team responsible for national security and policy matters, the letter said.

The FBI requested the materials of four additional senior members of the Trump transition team on 30 August, it said.

Mr Langhofer argued that, while such transition teams are involved in executive functions, they are considered private, non-profit organisations whose records are private and not subject to presidential records laws.