In German it's pretty much the same as in the USA ["Amerikaner" (American) is commonly understood as US citizen. Likewise, "Amerika" is commonly understood as USA.].
Though on the street "Ammi" is probably most common to refer to US citizens. It technically is not more than only using the first two letters of the original word and to give them a grammatic frame suitable to describe people. In practice, it carries a certain devaluation of Americans, fueled by stereo-types (Americans are dumb and fat / America is an evil imperialist in disguise). Though I feel like such Anti-American sentiment has decreased somewhat over the last years.