The International Monetary Fund says the United States must overhaul the housing finance system responsible for its financial crisis, possibly by privatizing Fannie Mae and Freddie Mac.

"The authorities should not delay efforts to create an action plan for the future," says its latest Global Financial Stability Report released Tuesday in South Africa. Fannie Mae and Freddie Mac "could be either privatized or converted to public utilities with an explicit (and explicitly funded) guarantee."

Some critics accuse the mortgage giants, which own or guarantee about half of all U.S. mortgages, of fueling the housing crisis.