WASHINGTON, April 15 (Reuters) - President Donald Trump on Wednesday threatened to fire Federal Reserve Chair Jerome Powell from his separate seat as a member of the U.S. central bank's Board of Governors if Powell does not vacate that post as well when his term as Fed chief ends on May 15, intensifying a complicated standoff that has upended the Fed's usually smooth transition of power.

Trump administration threats against Powell, including an ongoing criminal investigation, could delay Senate confirmation of Kevin Warsh as Trump's nominee to succeed Powell as Fed chief, but the president in a Fox Business interview doubled down on the probe as a way to prove Powell's "incompetence" and said if he doesn't leave altogether, "then I'll have to fire him."

"You want Jay Powell out of the way?" the president had been asked by Fox Business host Maria Bartiromo.

"If he's not leaving on time - I've held back firing him, I've wanted to fire him, but I hate to be controversial, you know. I want to be uncontroversial, but he will be fired," Trump responded. He gave no indication that U.S. Attorney for the District of Columbia Jeanine Pirro would back down from investigating a Fed building project the administration has criticized for cost overruns.

Trump's language underscored the stakes, and the potential complications the administration faces if Powell doesn't leave the Fed board.

With greater control of seats on the seven-member board, Warsh would have a freer hand in setting monetary policy and making other changes at the central bank that the administration might seek. Trump has appointed only three of the current members, and one of them, Fed Governor Stephen Miran, is in a seat whose term has already expired and, as it stands, would have to be vacated for Warsh to join.