Yes, you will need a US passport. US law does require that a US citizen traveling internationally carry a US passport. Specifically, it is unlawful for any citizen of the United States to depart from or enter, or attempt to depart from or enter, the United States unless he or she bears a valid United States passport.
In emergency situations a US passport can usually be issued in one to a few days, but it may require you to travel to a passport office that can process applications quickly. There are many passport acceptance agent offices around the country but less than 30 regional passport agency they can issue US passports within a few days.
If you are a US citizen trying to reenter the United States from an international location without a US passport, you may be delayed in your return while trying to prove you are a US citizen.
If you have any questions regarding immigration, please feel free to call me, immigration attorney Shawn Mesa at (813) 679-5780.
Disclaimer: This information is provided as a public service and not intended to establish an attorney-client relationship. Any reliance on this information is taken at your own risk.