Major US media say Justice Department Special Counsel Jack Smith, who brought two federal cases against Donald Trump, plans to resign before the president-elect takes office in January.

The New York Times and other outlets reported on Wednesday that Smith intends to get ahead of Trump's promise to fire him within "two seconds" of assuming the presidency.

Trump declared during a conservative radio show last month that he would immediately fire the special counsel if he returned to power.

Smith has been leading the investigation into the attack on the US Capitol in January 2021. Trump is facing criminal charges over the case, including conspiracy to defraud the US.

Smith has also been in charge of a case involving classified documents Trump allegedly retained at his home after leaving office.

Following Trump's victory in the US presidential election, the special counsel asked for court procedures to be paused in order to wind down his prosecution of the president-elect.

US media say Smith will move toward dropping the cases against Trump in line with long-held Justice Department policy that a sitting president cannot be criminally prosecuted.

They also say Smith plans to submit a final report summarizing his investigation and decisions to the current administration's attorney general.