The U.S. government Friday directed the government-back mortgage-finance companies to raise guarantee fees on single-family mortgages, as a fresh step to encourage greater participation in the mortgage market by private firms.

Fannie Mae and Freddie Mac, the two major U.S. mortgage-finance firms, have been required to increase guarantee fees on single- family mortgages by an average of 10 basis points, the U.S. Federal Housing Finance Agency (FHFA) said in a statement.

For loans exchanged for mortgage-backed securities (MBS), the increase will be effective with settlements starting Dec. 1, 2012. For loans sold for cash, the increases will take effect with commitments starting Nov. 1, 2012. The two government-supported companies will work directly with lenders to implement the changes, said the agency.

Meanwhile, the FHFA released its fourth annual report on single- family guarantee fees, saying that the average guarantee fees charged by Fannie Mae and Freddie Mac increased from 26 basis points in 2010 to 28 basis points in 2011.

The report also found that mortgages that posed higher credit risk, on average, were subsidized by lower-risk loans, and that a majority of the single-family mortgages acquired by Fannie Mae or Freddie Mac came from a small group of large lenders.

The fresh changes of guarantee fee will help address these two issues, noted the housing regulator. "These changes will move Fannie Mae and Freddie Mac pricing closer to the level one might expect to see if mortgage credit risk was borne solely by private capital," said Edward J. DeMarco, Acting Director of FHFA.

Due to sub-prime crisis broke out, the two mortgage financiers were placed into conservatorships on Sept. 6, 2008 and have since received more than 180 billion dollars in taxpayer support.

The U.S. government is trying to reduce its involvement in mortgage backing by simplifying and shrinking Fannie Mae and Freddie Mac's operations, and lure more private capital to house financing market. But these efforts are facing many challenges as they could lead to higher interest rates.

The two mortgage giants own or guarantee about half of all mortgages in the U.S., or nearly 31 million home loans. Along with other federal agencies, they provide more than 5.7 trillion dollars in funding for the U.S. mortgage market.