The outgoing head of the Internal Revenue Service apologized on Friday for "foolish mistakes" made at the U.S. tax agency which he said resulted from a heavy workload, not a partisan mission.

At a congressional hearing, acting IRS Commissioner Steven Miller said he did not find political motivation among the employees who were processing conservative groups' applications for tax-exempt status.

Miller was fired on Wednesday as President Barack Obama sought to stem criticism over IRS targeting of conservative groups for special scrutiny. On Thursday, Obama chose White House budget official Daniel Werfel to replace him as acting commissioner.

(Reporting by Laura MacInnis; Editing by Eric Beech)