The United States is expected to announce in the coming days that it will use offensive and defensive cyber capabilities on behalf of NATO if asked, a senior Pentagon official said, amid concerns about Russia's increasingly assertive use of its cyber capabilities.

The 29-nation NATO alliance recognized cyber as a domain of warfare, along with land, air and sea, in 2014, but has not outlined in detail what that entails.

"We will formally announce that the United States is prepared to offer NATO its cyber capabilities if asked," Katie Wheelbarger, the principal deputy assistant secretary of defense for international security affairs, told reporters during a trip to Europe by U.S. Defense Secretary Jim Mattis.

Wheelbarger said the United States will keep control of its people and capabilities but use them in support of NATO if asked. She added that it was a part of a British-led push to increase NATO's cyber capabilities.

In a recent summit, member nations said NATO would create a cyberspace operations center to coordinate NATO's cyber activities. NATO has also talked about integrating individual nations' cyber capabilities into alliance operations.

Last year, officials said the United States, Britain, Germany, Norway, Spain, Denmark and the Netherlands were drawing up cyber warfare principles to guide their militaries on what justifies deploying cyber attack weapons more broadly.

In Europe, the issue of deploying malware is sensitive because democratic governments do not want to be seen to be using the same tactics as an authoritarian regime.

Senior Baltic and British security officials say they have intelligence showing persistent Russian cyber hacks to try to bring down European energy and telecommunications networks, coupled with internet disinformation campaigns.

U.S. intelligence officials have found that in the campaign leading up to the 2016 U.S. presidential election, Russian hackers breached the Democratic National Committee and leaked confidential information.

"It sends a message primarily aimed at Russia," Wheelbarger said.

She added that the move would make clear that NATO is capable of countering Russian cyber efforts and would help in creating a more coherent cyber policy across the alliance.

"U.S. together with the United Kingdom clearly lead in the level and sophistication of capabilities and if used, they would likely lead to tactical success," said Klara Jordan, director of the Cyber Statecraft Initiative at the Atlantic Council think tank in Washington.

Banners with the NATO logo are pictured in front of the new NATO headquarters during a press tour of the facilities as the organization is moving from its old headquarters to the new building in Brussels, April 19, 2018. EMMANUEL DUNAND/AFP/Getty Images

Heather Conley, with the Center for Strategic and International Studies, said there was still a challenge in defining what the rules were when it came to cyber weapons.

"I think that the greater challenge, and we're certainly struggling with it here in the U.S., is what are the rules of engagement? What does the escalation ladder look like when one begins to use cyber offensive capabilities?" Conley, a former U.S. State Department official, said.

Last month the White House warned foreign hackers it would increase offensive measures as part of a new national cyber security strategy.

U.S. intelligence officials expect a flurry of digital attacks ahead of the Nov. 6 congressional elections.

(Reporting by Idrees Ali; Editing by James Dalgleish)