Special counsel Robert Mueller's office has reportedly asked the White House to hand over documents in relation to President Trump's former national security adviser Michael Flynn.

Mueller is looking into whether Flynn, whose consulting group Flynn Intel Group had worked with a Turkish-American businessman, was secretly paid by the Turkish government during the final months of the presidential campaign, New York Times reported Friday.

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The newspaper noted that it is the first known instance of the special counsel, which is probing ties between the Trump campaign and Russia, requesting documents from the White House.

It also shows that the special counsel probe has expanded into an examination of Flynn's financial deals, beyond disclosures about his conversations and business arrangements with Russian officials, the Times reported.

Mueller has impaneled a grand jury in Washington for the overarching Russia probe, The Wall Street Journal reported Thursday, a sign of the investigation entering a new phase.

Flynn resigned from the Trump administration in February after reports surfaced he had misled senior White House officials about his past conversations with Kremlin officials.

Flynn declined to comment for the Times report, while Trump's special counsel Ty Cobb stressed that the White House was cooperating with the probe.

"The White House will not be discussing any specific communications with the Special Counsel out of respect for the Special Counsel and his process. Beyond that, as I have stressed repeatedly, we continue to fully cooperate with the Special Counsel," Cobb said in a statement shared with The Hill.

– Jordan Fabian contributed

Updated: 10:45 p.m.