NEW YORK (Reuters) - An overhaul of U.S. mortgage agencies Fannie Mae and Freddie Mac might take longer than five years, depending on how severe the proposed changes are, an official at Federal Housing Finance Agency (FHFA) said on Monday.

If there is a “very radical” reform of the two government-sponsored enterprises, “we need more time” for the transition, Robert Ryan, special adviser and acting deputy director at the FHFA that regulates Fannie and Freddie, said at a conference sponsored by the Mortgage Bankers Association (MBA).

The Trump administration has set its sights on Fannie and Freddie. Earlier Monday, Treasury Secretary Steve Mnuchin repeated his call for the reform of Fannie and Freddie at the Milken Institute Global Conference and the government is expected to release a plan by 2018.

Fannie and Freddie guarantee U.S. home loans and repackage them into securities for sale to investors.

The government took the government-sponsored enterprises (GSEs) into conservatorship in 2008 during the global financial crisis after they suffered massive losses on bad mortgages due to the housing bust.

Under the conservatorship, profits from the two agencies have been going to the Treasury Department, even after they repaid the government for a combined $187 billion bailout.

Over the years, several ideas have been floated on how to deal with Fannie and Freddie, ranging from turning them into public companies again to phasing them out completely.

Last month, the MBA issued a report recommending the two agencies should be restructured as primary loan guarantors, together with a group of publicly held, regulated mortgage guarantors.

In 2016, Fannie and Freddie accounted for a combined 65 percent of new mortgage-backed securities involving single-family mortgages.

Outstanding single-family mortgages grew to nearly $10 trillion last year.

Because of Fannie and Freddie’s heavy footprint in the U.S. mortgage and bond markets, a major restructuring would likely be complicated, analysts said.

Trump’s proposed tax cuts and a major bond program to be launched also would need to be considered if Fannie and Freddie are drastically overhauled, according to FHFA’s Ryan.

“There are a lot of moving parts to it,” Ryan said. The transition under any GSE reform “should not be rushed.”