WASHINGTON (AP) -- The White House said Tuesday that a Florida church's threat to burn copies of the Muslim holy book could endanger U.S. troops abroad, while the State Department denounced the plan as "un-American" and said it would put American diplomats and travelers at risk. White House spokesman Robert Gibbs noted the U.S. commander in Afghanistan, Gen. David Petraeus, has warned that images of a burning Quran would be used by extremists to incite violence.