The United States and its NATO allies are preparing militarily for the prospect that their rift with Russia could outlast President Vladimir Putin, U.S. Defense Secretary Ash Carter said on Sunday.

Russia's intervention in Ukraine has put NATO allies in eastern Europe on edge and triggered a series of military moves by the alliance, including an acceleration of exercises and the creation of a NATO rapid response force.

Carter, speaking at the start of a weeklong trip to Europe, said the United States hoped Russia would return to a forward-looking course and noted areas of diplomatic cooperation with Moscow, including talks over Iran's nuclear program.