The special counsel probing whether US President Donald Trump's campaign colluded with Russia has unlawfully received tens of thousands of emails, a lawyer for Trump's transition alleges, according to US media.

In a letter to congressional committees, Kory Langhofer says the General Services Administration (GSA) "unlawfully produced" the private materials, including privileged communications which special counsel Robert Mueller then used as part of his Russia probe, Axios news site reported on Saturday.

GSA is the government agency which supports presidential transitions.

The accusations by Langhofer, representing Trump's transition team, are the latest by Republicans seeking to raise doubts about the credibility of Mueller's investigation, which has already led to charges against Trump's former national security advisor and three other people linked to the presidential campaign.

Langhofer wrote that Mueller's office "received from the GSA tens of thousands of emails, including a very significant volume of privileged material," according to a copy of the letter published by Politico.

The letter suggests that a warrant should have been obtained for such materials, and says that the incident undermines the Presidential Transition Act.

"This is another attempt to discredit Mueller as his probe tightens," California Democratic Congressman Eric Swalwell said on Twitter in response to the Langhofer accusations.

Some members of Trump's Republican party have already urged that a second special counsel be named to investigate Mueller's operation.

The special counsel's critics have seized upon the case of Peter Strzok, a senior FBI agent whom Mueller removed from his team for sending text messages critical of Trump.

In addition to whether Trump's campaign team colluded with Russia to help him win last year's election, the special counsel is also looking at possible obstruction of justice, after Trump fired FBI director James Comey.

Trump has repeatedly denied that there was any collusion.