also found this on the Embasy's FAQ page:
I am an America citizen, but also have a foreign passport. Can I use my foreign passport to travel to the United States?
No. Under U.S. law, U.S. citizens must be in possession of a valid U.S. passport to enter or leave the United States. This is true even if you hold a passport from another country. If your U.S. passport has been lost or stolen, or if it has expired, you must apply to replace it before traveling to the United States.