Treasury Secretary Janet Yellen, in a letter to Congress on Thursday, said the U.S. has reached its debt limit, and has begun resorting to "extraordinary measures"

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Yellen had already previously warned the U.S. would reach its debt limit on Thursday but had said the Treasury Department

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The special measures being undertaken, including suspending investments in the Civil Service Retirement Fund, are essentially accounting maneuvers

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The U.S. economy runs a deficit, hence it needs to borrow money to pay its bills. Currently the U.S. can only borrow up to about $31.4 trillion.

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Raising the debt limit used to be a routine exercise for Congress but it has become increasingly fraught. A potentially huge political fight looms this year

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That's a big concern given that a U.S. debt default would have potentially huge consequences for the economy and markets, raising doubt about the country's creditworthiness.

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Analysts had previously estimated the debt limit could be reached as early as June with the measures Treasury has taken

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The period of time that extraordinary measures may last is subject to considerable uncertainty," Yellen wrote.

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I respectfully urge Congress to act promptly to protect the full faith and credit of the United States."

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The U.S. almost defaulted on its debt in 2011 when political brinkmanship between House Republicans and then-President Obama

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