The U.S. mortgage finance system should be revamped and any government guarantees should be clearly defined and the risk priced accordingly, Treasury Secretary Timothy Geithner plans to tell Congress on Tuesday.

"Private gains can no longer be supported by the umbrella of public protection, capital standards must be higher and excessive risk-taking must be appropriately restrained," Geithner said in testimony prepared for delivery to the House of Representatives Financial Services Committee.

A copy of the testimony was obtained by Reuters on Monday.

Geithner's comments mark a first step in what is expected to be a long process of overhauling the way U.S. homes are financed.

Geithner's predecessor, Henry Paulson, in 2008 effectively nationalized Fannie Mae and Freddie Mac, which own or guarantee about half of all U.S. residential mortgages.

The Treasury chief said any effort to overhaul the housing finance system should ensure that mortgage credit is widely available and should promote affordable housing, consumer protection and financial stability.

While the quasi-governmental status of Fannie Mae and Freddie Mac turned out to be a mistake, Geithner said "there should be no uncertainty" about the government's support of the two entities during the transition period to a new system.