Special Counsel Robert Mueller warned he could order Donald Trump to testify as part of a probe into alleged Russian election meddling, US media report.
It is believed to be the first time the special counsel has raised the possibility of issuing a subpoena.
Mr Trump has repeatedly denied collusion, and Russia has also denied the allegations.
Meanwhile, the lawyer who has been Mr Trump’s main legal representative in the probe is to step down.
The White House said on Wednesday that Ty Cobb had been considering retirement for some time and had decided to go at the end of this month.
Mr Mueller suggested the threat of a subpoena, a court order forcing a witness to appear to give testimony, during talks with Mr Trump’s lawyers in March.
It came after Mr Trump’s lawyers insisted during the meeting that the president was under no obligation to face questions by federal investigators in relation to the Russia inquiry, the Washington Post reported.