WASHINGTON — Tens of thousands of people may have taken advantage of the first-time home buyer tax credit to defraud the government, an IRS watchdog office said Thursday, in testimony that could jeopardize efforts to extend the popular program.

Treasury Inspector General for Tax Administration J. Russell George told a House panel that more than 19,000 people filed 2008 tax returns claiming the credit for homes they had not yet purchased. Russell said his office had identified another $500 million in claims, by some 74,000 taxpayers, where there were indications of prior home ownership.

He told a House Ways and Means oversight subcommittee that they also found 580 taxpayers under the age of 18 who claimed $4 million in first-time home buyer credit. One was 4 years old.

George said that while the IRS has since taken steps to tighten oversight, "some key controls were missing to prevent an individual from erroneously or fraudulently claiming the credit."

The home buyer credit was a key element of the $787 billion stimulus package enacted last February. Under the measure, low- and middle-income first-time home buyers purchasing a home between Jan. 1 and Nov. 30 of this year could claim a credit of up to $8,000 on their 2008 or 2009 income tax return.

The Internal Revenue Service says it has processed claims from more than 1.5 million individuals or families. The General Accountability Office, in a report to the subcommittee, said that represented about $10 billion in tax revenue.

With the program scheduled to expire in a month and the housing market's recovery still shaky, there have been various proposals in Congress to extend and expand it.