By Phil Stewart
PALO ALTO, California (Reuters) - The United States on Thursday disclosed a cyber intrusion this year by Russian hackers who accessed an unclassified U.S. military network, in a episode Defense Secretary Ash Carter said showed the growing threat and the improving U.S. ability to respond.
Carter cited the newly declassified incident during an address at Stanford University, in which he also warned the Pentagon was ready to help defend America's networks and to use cyber weaponry, if needed.
The doctrine was articulated in a new Pentagon cyber strategy unveiled on Thursday. Reuters obtained a copy on Wednesday.
"Adversaries should know that our preference for deterrence and our defensive posture don't diminish our willingness to use cyber options if necessary," Carter said in prepared remarks.
Carter said that sensors guarding the Pentagon's unclassified networks detected the intrusion by Russian hackers, who discovered an old vulnerability that had not been patched.
"While it's worrisome they achieved some unauthorized access to our unclassified network, we quickly identified the compromise and had a crack team of incident responders hunting the intruders within 24 hours," Carter said.
"After learning valuable information about their tactics, we analyzed their network activity, associated it with Russia, and then quickly kicked them off the network, in a way that minimized their chances of returning," he added.
The Pentagon's new cyber strategy document singles out Russia and China, saying both had developed advanced cyber capabilities and strategies.
"Russian actors are stealthy in their cyber tradecraft and their intentions are sometimes difficult to discern," the strategy document said.
Iran and North Korea had "less developed cyber capabilities" but overt hostile intent toward U.S. interests, it said.
On Thursday, Carter stressed the U.S. military needed closer cooperation with California's Silicon Valley, particularly after high-profile attacks on companies like Sony Pictures Entertainment.
"As tech companies see every day, the cyber threat against U.S. interests is increasing in severity and sophistication," he said. "While the North Korean cyberattack on Sony was the most destructive on a U.S. entity so far, this threat affects us all."
Carter's trip to Stanford, in the heart of Silicon Valley, is the latest example of the U.S. government's efforts to improve relationships with tech companies after damaging revelations over digital surveillance by former National Security Agency contractor Edward Snowden.
Carter acknowledged part of his job will be overcoming those concerns.
"There are also really great opportunities to be seized through a new level of partnership between the Pentagon and Silicon Valley," he said.
(Reporting by Phil Stewart; Editing by Susan Heavey and Doina Chiacu)