U.S. Defense Secretary Ashton Carter has accused Russia of endangering world order, citing its incursions in Ukraine and loose talk about nuclear weapons.
Carter said Russia is undertaking "challenging activities" at sea, in the air, in space, and in cyberspace.
Carter said the U.S. defense establishment is searching for creative ways to deter Russian aggression and protect U.S. allies.
His remarks were made on November 7 at the Ronald Reagan Presidential Library in Simi Valley, California, after eight days of travel in Asia.
"Most disturbing, Moscow's nuclear saber-rattling raises questions about Russian leaders' commitment to strategic stability, their respect for norms against the use of nuclear weapons, and whether they respect the profound caution nuclear-age leaders showed with regard to the brandishing of nuclear weapons," he said.
Carter also expressed concern about China's expanding influence and growing military might. But he reserved his stronger words for Russia.
Carter cited several pillars of the international order that he argued should be defended and strengthened: peaceful resolution of disputes, freedom from coercion, respect for state sovereignty, and freedom of navigation.