WASHINGTON, May 15 (Reuters) - The Federal Reserve Board on Friday said it has named Jerome Powell as chair pro tempore until Kevin Warsh is sworn in as the new head of the U.S. central bank.

-- Warsh is expected to be sworn in soon by U.S. President Donald Trump but no date has been announced.

-- Powell's eight-year run as Fed leader formally expired on Friday.

-- Fed board members Stephen Miran and Michelle Bowman said in a joint statement they did not support the measure that keeps Powell as chair because it does not include a fixed time period for his temporary status.

-- Although Powell is stepping down as chair, he has said he will stay on the Fed's Board of Governors until he is satisfied the Trump administration has ended its criminal probe of him.