WASHINGTON—Speaker Nancy Pelosi said the House would move ahead with an “official” impeachment effort after reports that President Trump withheld aid to Ukraine while he was pressing the country to investigate Democratic presidential candidate Joe Biden and his son.

After months of resisting an impeachment investigation, Mrs. Pelosi said at the end of a day of meetings with House Democrats that she was directing six House committees already investigating Mr. Trump to continue their probes “under that umbrella of impeachment...