WASHINGTON — Fannie Mae on Monday posted a $29 billion loss in the third quarter as it took a massive tax-related charge, and said it may have to tap the government's $100 billion lifeline as early as next year.

The mortgage finance company, seized by federal regulators more than two months ago, posted a loss of $13 per share for the July-September quarter, mainly due to a $21.4 billion non-cash charge to reduce the value of tax assets. That compares to a loss of $1.4 billion, or $1.56 a share, in the year-ago period. Analysts surveyed by Thomson Reuters had expected a loss of $1.60 per share.

Fannie Mae's net worth — the value of its assets minus the value of its liabilities — fell to $9.4 billion at the end of September from $44.1 billion at the end of last year. If that number turns negative, Fannie Mae said it would be required to obtain funding from the Treasury Department.

The ultimate bill for taxpayers may depend on whether the government, under President-elect Barack Obama, uses Fannie Mae and its sibling company Freddie Mac as a way to alleviate the foreclosure crisis by aggressively modifying or refinancing loans.

"They're no longer being run for profit," said Fox-Pitt Kelton analyst Howard Shapiro.

Washington-based Fannie Mae posted a loss of $13 per share for the July-September quarter, mainly due to a $21.4 billion non-cash charge to reduce the value of a tax asset and $9.2 billion in expenses resulting from falling home prices and surging defaults.

Fannie Mae, which has bled $33.5 billion in red ink so far this year, is now run by CEO Herbert Allison, formerly chairman and chief executive of retirement fund manager TIAA-CREF. Fannie Mae's former top executive, Daniel Mudd, was ousted as part of the government takeover.

Fannie Mae and Freddie Mac, which own or guarantee around half of U.S. home loans, operate in a conservatorship that enables the government to inject up to $100 billion in each company in exchange for ownership stakes of almost 80 percent. They also are facing a federal grand jury investigation into their accounting practices.

Fannie Mae said last month that it would change its accounting for its deferred-tax assets, which can emerge from operating losses, and can be used to reduce future tax expenses. Companies must be able to show they will be profitable if they intend to use the tax assets for earnings in later periods.

Fannie Mae said in the SEC filing that it made the accounting change due to "the uncertainty of future market conditions on our results of operations and the uncertainty surrounding our future business model." Even after the change, Fannie Mae still has $4.6 billion in tax assets on its books, but warned it may have to reduce their value.

Meanwhile, the U.S. housing market continued to decline. Fannie Mae posted $9.2 billion in credit losses, up from $1.2 billion in the quarter a year earlier and $5.4 billion in the second quarter. Delinquent loans rose to 1.7 percent of all single-family loans, up from 1.4 percent in the second quarter and 0.8 percent last fall.

Fannie Mae owned more than 67,500 foreclosed properties at the end of September, up 25 percent from the end of June.

Shares rose 1 cent to 75 cents in morning trading.