U.S. Defense Secretary Ash Carter is strongly criticizing Russia for what he says is Moscow's "clear ambition to erode the principled international order" through coercion and aggression.

Carter used a speech Wednesday to students at Oxford University to blast what he calls Russia's "unprofessional behavior" in Ukraine, Syria and cyberspace. He's accusing Moscow of nuclear "saber rattling."

Carter says Russia seeks to sow instability beyond its borders, including in the Middle East. He's also urging Russia to take a more constructive role in Syria to arrange a lasting cease-fire.

He cites progress the countries made together in the aftermath of the Cold War, but says Russia now "appears driven by misguided ambition and misplaced fear."