Warren Buffett acknowledged this week Berkshire Hathaway has talked to five big financial firms about buying lucrative tax credits against expected opposition from the U.S. Treasury.

Buffett told CNBC he can't name any names due to confidentiality agreements, but notes that Berkshire has been buying tax credits since 1990 and will continue to do so in the future.

Berkshire Hathaway has teamed with Goldman Sachs in the investment bank's attempt to buy $3 billion of tax credits from taxpayer-owned mortgage firm Fannie Mae, The Wall Street Journal reported.

The paper, citing an unnamed investor who had analyzed a similar possible investment, said a deal could could produce annualized returns of at least 30 percent for Berkshire and Goldman.

The paper said a Berkshire Hathaway representative did not have an immediate comment on the report, and that Goldman Sachs and Fannie Mae declined to comment. Treasury officials told the paper they are still reviewing the proposal.

A recent Treasury Department analysis found such a deal would not be advantageous for taxpayers.

Treasury's finding raises the odds that the Obama administration will seek to substantially alter or even block the deal, the latest example of the government's growing role in overseeing negotiations between financial companies.

The deal comes at a sensitive time, when resentment remains over the ability of Wall Street firms like Goldman to rebound so quickly, racking up healthy profits after taking government bailouts at the height of the financial crisis.

Treasury has a say in the matter because the federal government seized Fannie Mae in September 2008 and has since pumped more than $45 billion into the company to keep it afloat. While technically a public company listed on the New York Stock Exchange, all of Fannie Mae's major business decisions are reviewed by federal officials.

The discussions focus on tax credits accumulated by companies that invest in low-income rental properties. Companies can deduct the value of the credits from their income tax bill or sell them to other firms.

But these credits have lost much of their value over the past 18 months as the economic crisis has worsened and financial markets have plummeted. Firms didn't plan on making much profit, so they didn't need tax credits.

The declining value of its credits has forced Fannie Mae to record even greater losses, requiring more government capital to stay solvent.