WASHINGTON — Russia's actions in Ukraine have created a "new reality" for the American military and its NATO allies, said US Defense Secretary Ash Carter said on Wednesday.

Speaking at the AUSA conference in DC here, Carter pledged to use a "strong and balanced approach" to addressing Russia's actions, but seemed to acknowledge that the relationship between the NATO alliance and the government of Russian President Vladimir Putin has irrevocably changed since the invasion of Ukraine in 2014 last year.

"This is the new reality for us, strategically, and it looks like it's here to stay," Carter said, in a line that was not included in his prepared remarks.

That new reality requires a new "playbook" for Europe, said Carter, with the Army firmly at the center.

"The 20th century playbook was successful in creating a Europe whole, free, and at peace, but the same playbook would not be matched to the 21st century," Carter said in his comments. "We must write a new playbook, which includes preparing to counter new challenges like cyber and hybrid warfare, better integrating conventional and nuclear deterrence, as well as adjusting our posture and presence to adapt and respond to new challenges and threats.

"Russia has the opportunity to change course and rejoin the track toward a political transition in Damascus. I don't know if they will," Carter addedsaid. "From the Kamchatka Peninsula through South Asia, into the Caucasus and around to the Baltics, Russia has continued to wrap itself in a shroud of isolation. And only the Kremlin can decide to change that."

Carter also used the speech to hit back at what he called "unprofessional behavior" from Russian pilots who have begun operating in Syria.

"They've violated Turkish airspace — which, as we strongly affirmed last week in Brussels, is NATO airspace," he notedsaid. "They've shot cruise missiles from a ship in the Caspian Sea without warning. They've come within just a few miles of one of our unmanned aerial vehicles."

While pledging that the US "have[has] not, and will not, agree to cooperate with Russia so long as they continue to pursue this misguided strategy," Carter did touch on the ongoing discussions to deconflict the airspace above Syria.

Those discussions, ongoing as of Wednesday, seek to avoid a potential accident between the US-led coalition and Russian jets. There have been multiple instances over the last two weeks of Russian and US jets entering flying in relatively close proximity.

Twitter: @AaronMehta