President Obama and German Chancellor Angela Merkel condemned Russia's annexation of Crimea in a call Tuesday.
The leaders said the move was "a violation of international law, and noted there would be costs," according to a White House readout.
Voters in Crimea on Sunday voted overwhelmingly to secede from Ukraine in a referendum denounced by the U.S. and its allies as illegal under Ukrainian and international law.
The White House said Obama and Merkel agreed to continue to underscore to Russian President Putin that a diplomatic solution remains possible.
Germany will be among the Group of Seven countries meeting with the United States on the sidelines of an upcoming nuclear security summit in The Hague next week.