Defense Secretary Ash Carter is criticizing Russia for aggression in Europe and is promising to continue a military buildup to deter war on NATO's eastern flank.

In remarks Tuesday at a ceremony installing a new commander of U.S. Forces in Europe, Carter said he is particularly troubled by what he called Russian "nuclear saber-rattling."

Carter also held out hope for a more cooperative relationship with Russia. He's said it's up to Moscow, which should know that Washington does not seek war.

Army Gen. Curtis Scaparrotti took command of U.S. European Command, succeeding Air Force Gen. Philip Breedlove, who is retiring.