The regulator overseeing Fannie Mae and Freddie Mac announced Monday one of the most concrete efforts to date for building a new infrastructure that could ultimately replace the government-controlled mortgage companies.

Edward DeMarco, acting director of the Federal Housing Finance Agency, said the agency would begin forming a company that would consolidate some of the "back-office" functions currently replicated individually by each firm. The company would have its own chief executive and board and for now would be jointly owned by Fannie and Freddie, Mr. DeMarco said in a speech Monday before the National Association of Business Economics in Washington, D.C.

Fannie and Freddie have operated independently for decades in competition with each other, but last year the FHFA began working with them to create a single platform in which home loans could be packaged into securities.

The two mortgage giants collapsed as the housing sector deteriorated five years ago, and their rescues have cost taxpayers $131 billion so far. They were taken over by the U.S. Treasury in 2008, and the FHFA was tasked with conserving the firms' assets until Congress and the White House decided what to do with them.

Until now, few steps have been taken toward any overhaul. Fannie and Freddie, together with federal agencies, are responsible today for backing nearly nine of 10 new mortgages, with taxpayers on the hook if those loans default.