The U.S. Treasury plans to kick off a fresh $15 billion liquidity and mortgage bond purchase program to assist state finance agencies providing mortgages to low-income borrowers, Bloomberg said, citing a person familiar with the matter.

The program, administered by mortgage giants Fannie Mae and Freddie Mac, will buy up to $20 billion in tax-exempt mortgage bonds issued by the state-sponsored housing agencies through the end of the year, and is expected to inject additional liquidity for as long as three years, the agency said.

Bloomberg also said the Treasury initiative, expected to be announced by Sept 30, will provide sufficient financing to restart and fund state home loan programs through end-2010, adding, Freddie Mac and Fannie Mae will also purchase the bonds.

The department will provide a federal backstop for a number of liquidity facilities, to lower funding costs, the agency said.

The Treasury did not immediately respond to a Reuters email seeking comment outside regular business hours.

Sales of previously owned U.S. homes fell unexpectedly in August, a survey showed last week, a minor setback for the housing market's recovery from a three-year slump.

(Reporting by Biswarup Gooptu in Bangalore; Editing by Kim Coghill)