WASHINGTON — President Trump promised on Friday that he would unveil a “massive” tax cut for Americans next week, vowing a “big announcement on Wednesday,” but he revealed no details about what is certain to be an enormously complicated effort to overhaul the nation’s tax code.

Mr. Trump offered his tax tease in an interview and again during remarks at the Treasury Department on Friday afternoon as he raced to stack up legislative accomplishments before his 100th day in office at the end of next week.

His announcement surprised Capitol Hill and left Mr. Trump’s own Treasury officials speechless as he arrived at the Treasury offices to sign directives to roll back Obama-era tax rules and financial regulations. Earlier in the day, when reporters asked Steven Mnuchin, the Treasury secretary, how far away a tax overhaul proposal was, he said he could not give an answer. “Tax reform is way too complicated,” he said.

Mr. Trump told The Associated Press in the interview that his tax reductions would be “bigger, I believe, than any tax cut ever.” But he faces an enormous fight among clashing vested interests as Congress tries to rewrite the tax code.