The United States is adapting its “operational posture” to counter Russian aggression, Defense Secretary Ashton Carter said Saturday.

Carter on Saturday accused Russia of endangering world order, citing its incursions in Ukraine and loose talk about nuclear weapons, and said the U.S. defense establishment is searching for creative ways to deter Russian aggressive and protect US allies.

“We are adapting our operational posture and contingency plans as we -– on our own and with allies -– work to deter Russia’s aggression, and to help reduce the vulnerability of allies and partners,” Carter told a defense forum at the Ronald Reagan Presidential Library in California.

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Carter said the United States was modernizing its nuclear arsenal, investing in new technologies such as drones and a new long-range bomber, as well as lasers, an electromagnetic railgun and new systems for electronic warfare.

The defense chief hinted at additional new weapons that would be “surprising ones I really can’t describe here.”

Additionally, “we’re updating and advancing our operational plans for deterrence and defense given Russia’s changed behavior,” Carter said.

Carter also expressed concern about China’s expanding influence and growing military might. But he reserved his stronger words for Russia.

Carter said Russia is undertaking “challenging activities” at sea, in the air, in space and in cyberspace.

“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.

His remarks were perhaps the strongest he has expressed about America’s former Cold War foe.

“We do not seek a cold, let alone a hot, war with Russia,” he said. “We do not seek to make Russia an enemy. But make no mistake; the United States will defend our interests, our allies, the principled international order, and the positive future it affords us all.”

The backdrop to Carter’s remarks is the reality that after more than two decades of dominating great-power relations, the United States is seeing Russia reassert itself and China expand its military influence beyond its own shores. Together these trends are testing American preeminence and its stewardship of the world order.

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.

“Some actors appear intent on eroding these principles and undercutting the international order that helps enforce them,” he said. “Terror elements like ISIL, of course, stand entirely opposed to our values. But other challenges are more complicated, and given their size and capabilities, potentially more damaging.”

“Of course, neither Russia nor China can overturn that order,” he said. “But both present different challenges for it.”

He accused Russia of stirring trouble in Europe and the Middle East.

“In Europe, Russia has been violating sovereignty in Ukraine and Georgia and actively trying to intimidate the Baltic states,” he said. “Meanwhile, in Syria, Russia is throwing gasoline on an already dangerous fire, prolonging a civil war that fuels the very extremism Russia claims to oppose.”

Carter made clear that Russia is at the forefront of Washington’s concern about evolving security threats.

Russia under President Vladimir Putin is challenging the US in many arenas, including the Arctic, where last year Moscow said it was reopening 10 former Soviet-era military bases along the Arctic seaboard that were closed after the Cold War ended in 1991. Russia also is flying more long-range air patrols off US shores.

Carter left open the possibility that Russia’s role in Syria could evolve into one the US can embrace.

“It is possible – we’ll see – Russia may play a constructive role in resolving the civil war,” he said.

In a question-and-answer session with his audience, Carter said he believes Putin “hasn’t thought through very thoroughly” his objectives in Syria. He called the Russian approach there “way off track.”

In his speech, Carter said the US will take a balanced approach by working with Moscow when productive and appropriate.

Carter was addressing key US politicians and figures from the defense sector at the Reagan National Defense Forum, hosted by the Reagan library in Simi Valley.