Treasury Secretary Janet Yellen speaks to reporters during her trip to China.Pedro Pardo/AFP/Getty ImagesTreasury Secretary Janet Yellen wrote a letter to House Speaker Mike Johnson Friday.In it, she warned that the US would breach its debt limit as soon as January 14 unless Congress acted.The Treasury Department will begin taking "extraordinary measures" to avoid defaulting on debt.The US will need to take "extraordinary measures" to avoid defaulting on its debt as early as January 14, Treasury Secretary Janet Yellen warned in a letter on Friday.In the letter addressed to House Speaker Mike Johnson, Yellen wrote that although the outstanding debt subject to the limit is expected to drop by approximately $54 billion when the new debt limit is established on January 2, it won't be long before the US does hit its limit.She said the Treasury Department will likely need to start making accounting manuevers between January 14 and 23 to prevent the US from defaulting.The debt limit is the mechanism that restricts the amount of money the US can borrow.