By ROBERT BURNS, AP National Security Writer

SIMI VALLEY, Calif. (AP) — Defense Secretary Ash 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 aggression and protect U.S. allies.

Carter also expressed concern about China's expanding influence and growing military might, but he reserved his stronger words for Russia in his remarks to the Reagan National Defense Forum. National security experts and defense officials attended the gathering at the Ronald Reagan Presidential Library.

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, returning from eight days of travel in Asia, 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.

"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," he said.

Russia under President Vladimir Putin is challenging the U.S. 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 U.S. shores.

Carter left open the possibility that Russia's role in Syria could evolve into one the U.S. 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 U.S. will take a balanced approach by working with Moscow when productive and appropriate.

As Russia makes what Carter characterized as threatening statements about its potential use of nuclear weapons, the U.S. is modernizing its entire nuclear arsenal — not only the submarines, bomber aircraft and land-based missiles that are armed with long-range nuclear weapons, but also the weapons themselves.

"We're investing in the technologies that are most relevant to Russia's provocations, such as new unmanned systems, a new long-range bomber, and innovation in technologies like the electromagnetic railgun, lasers and new systems for electronic warfare, space and cyberspace, including a few surprising ones I really can't describe here," he said.

Carter said China is the single most influential player in Asia's future, and he noted that earlier this week he went aboard an American aircraft carrier in the South China Sea to demonstrate U.S. commitment to freedom of navigation. The U.S. objects to China's claims of territorial limits around disputed artificial islands there.

"As a rising power, it's to be expected that China will have growing ambitions and a modernizing military," he said. "But how China behaves will be the true test of its commitment to peace and security."

He said the U.S. has been shifting its focus toward the Asia-Pacific, including sending its best naval and other military weapons, ships and equipment to that region.

"We are also changing fundamentally our operational plans and approaches to deter aggression, fulfill our statutory obligations to Taiwan, defend allies, and prepare for a wider range of contingencies in the region than we have traditionally," he said.