Federal Prosecutor Monika Harms said the three suspects intended to attack institutions and establishments frequented by Americans in Germany, including discos, pubs and airports. Her office said the plan was to set off car bombs.
"We were able to succeed in recognizing and preventing the most serious and massive bombings," Harms said at a news conference. She declined to name specific targets.
"There was an imminent threat," Defense Minister Franz Josef Jung told ARD broadcaster.
A senior State Department official said German investigators determined that Frankfurt International Airport and the nearby U.S. Ramstein Air Base were the primary targets, although other sites may have under consideration.
Ramstein AB is about 5km from Landstuhl Regional Medical Center.
Anti-terror authorities have been watching the suspects since late 2006 when they were observed carrying out surveillance of U.S. military facilities in Hanau, near Frankfurt.