By Jan Schwartz and Maria Sheahan HAMBURG/FRANKFURT (Reuters) – Hamburg has told Uber, a U.S. car service whose smartphone app summons rides at the touch of a button, to stop operating in the German city, adding to resistance the company has faced globally from local regulators and taxi drivers. San Francisco-based Uber Technologies Inc, valued at $18.2 billion in a fundraising last month just four years since its 2010 launch, has touched a raw nerve by threatening to open up a traditionally tightly controlled and licensed market. Uber customers order and pay for a taxi with its application on their smartphones. Instead of having taxis prowl city streets looking for customers, Uber allows smartphone users to summon a nearby car to pick them up.