Joint Chiefs Chairman Gen. Martin Dempsey urged his Russian counterpart to show "restraint" in Ukraine during a phone call on Wednesday.
Speaking at a budget hearing, Dempsey highlighted "the alarming progression of events in Ukraine over the past few days" and accused Moscow of "aggression."
He added: "I spoke this morning with my Russian counterpart, General [Valery] Gerasimov. I conveyed to him the degree to which Russia's territorial aggression has been repudiated globally. I urged continued restraint in the days ahead, in order to preserve room for a diplomatic solution."
Meanwhile, Defense Secretary Chuck Hagel told the Senate Armed Services Committee on Wednesday that the U.S. was stepping up joint aviation training with Polish forces.
The Pentagon also is increasing American participation in NATO's air policing mission in its Baltic countries, Hagel said.
Courtney Kube of NBC News and The Associated Press contributed to this report.