The New York Times reported on Wednesday that Mueller's office had sent the White House a document outlining 13 areas of interest about which investigators want additional documentation.

Mueller's investigation - and two congressional probes - are looking into US intelligence agencies' claimsthat Russia worked to help Republican Donald Trump's election victory last November.

Mueller's office is reportedly expected to interview at least half a dozen current and former Trump aides.

Lawyers for the White House will cooperate with the document requests, The Associated Press reported, citing unnamed sources.

The full scope of the investigation is not yet clear, but the requests indicate some of the areas that Mueller and his team of prosecutors intend to look into.

Trump Jnr meeting tops the list

The June 2016 meeting at Trump Tower that Donald Trump Jr. attended with a Russian lawyer - as well as the administration's response to it – is a key event for the Mueller team, AP said.

A statement given to journalists in July - which the White House said Trump had had a hand in drafting - said the meeting was primarily about a US program for adopting Russian children.

However, emails released days later by Trump Jr. show that he arranged the meeting expecting to receive damaging information about Trump's opponent, Hillary Clinton.

Ex-FBI chief James Comey's firing in May is reportedly also being investigated for potential obstruction of justice.

Deputy Attorney General Rod Rosenstein - who appointed Mueller as special counsel - has been questioned by investigators over the circumstances of the firing, the New York Times reported.

Investigators in July raided the home of Trump's former campaign chairman, Paul Manafort, in a search of tax and banking records. In recent months they have also served subpoenas related to Manafort's and Flynn's business dealings. The 2016 meeting at Trump Tower was also attended by Trump's son-in-law Jared Kushner and Manafort.

The Washington Post reported that the list includes an Oval Office meeting Trump had in May with Russian Foreign Minister Sergei Lavrov the day after he fired Comey.

Flynn 'compromised'

Investigators also are interested in White House actions involving Flynn, including what officials knew about an FBI investigation into him and how they responded.

Flynn was forced out as national security adviser in February after White House officials said he had misled them about his conversations with the Russian ambassador to the US, Sergey Kislak.

Former Deputy Attorney General Sally Yates has said she warned White House counsel Don McGahn in January that deception left Flynn and the White House in a compromised position.

The conversation took place two days after FBI agents had interviewed Flynn, but Flynn was not asked to resign until several weeks later, following news reports that said he had discussed sanctions during the transition period with the ambassador, Kislyak.

Former FBI Director James Comey has said that Flynn was facing an FBI criminal investigation into "his statements in connection with the Russian contacts and the contacts themselves."

Comey has also said that Trump, in a private Oval Office meeting in February, told him that he hoped he would end the FBI investigation into Flynn, an allegation that Trump has denied.

Mueller's team of investigators includes prosecutors with experience in organized crime, national security and complex financial fraud cases. 

Among the aides expected to be interviewed in coming weeks are McGahn, former press secretary Sean Spicer and former chief of staff Reince Priebus.

Ty Cobb, the White House lawyer handling requests from Mueller's office, told the news agency Reuters: "The White House remains committed to cooperating fully with the special counsel."

jbh/rc (AP, Reuters, AFP)