BERLIN (Reuters) - NATO should defend Sweden and Finland in the event of an armed aggression, although neither country is an alliance member, a senior NATO official said on Wednesday at an event that underscored growing concerns about Russia’s military buildup.

FILE PHOTO: The NATO flag is seen through barbed wire as it flies in front of the new NATO Headquarters in Brussels, Belgium May 24, 2017. REUTERS/Christian Hartmann

Commodore Hans Helseth, special adviser to the NATO Joint Warfare Center in Norway, said the two nations’ growing ties to the western alliance increased their risk and NATO had a moral obligation to come to their assistance if they were attacked.

“Their non-membership actually lowers the threshold of an armed aggression against the two countries. In my opinion, NATO simply has for moral reasons and other reasons to come to the two countries’ assistance,” Helseth said at the Berlin Security Conference.

He said NATO remained open to welcoming both nations as members if they chose to join.

Finland, Sweden, Denmark, Norway and Iceland agreed this month to step up defense cooperation and exchange more air surveillance information, part of a broader effort to build up their defenses to counter Russian activities.

Officials from both Finland and Sweden spoke at the conference about their concerns about Russia’s military buildup in the Arctic and northern Europe in general, citing its 2014 annexation of the Crimea region of Ukraine as a wakeup call.

The two countries take part in NATO military drills, but they are not covered by Article V of NATO’s foundational treaty which says an attack on one member is an attack on all. Both countries know any move to join the alliance would create a backlash in Moscow, which opposes any expansion of NATO.

A senior Russian foreign ministry official on Tuesday called for efforts to rebuild ties with European countries, including more communication among military experts.

But Helseth said there could be “no return to business as usual” with Russia given its annexation of Crimea and its role in supporting violence in eastern Ukraine, as well as its support of Syrian President Bashar al-Assad and use of “unfriendly attacks in the cyber domain”.

Top military leaders from NATO and member countries spoke at the two-day event about their efforts to build up a bigger deterrent to any military aggression by Russia.

U.S. Lieutenant General Ben Hodges, who heads U.S. Army forces in Europe, said Russia had missed a big opportunity to mend ties with the West by not being more transparent about military exercises this year.

U.S. and NATO officials say Russian forces far exceeded the 13,000 threshold beyond which a country is obligated to invite military observers. Russia says western officials are exaggerating the scope of the exercises.

Hodges said western nations had achieved an unprecedented level of information-sharing during the exercise, and said he hoped that would continue even as NATO expands its own capabilities to rapidly move forces to the eastern flank.

He cited concerns about recent activities by Russia, including its purchase of 2,000 vehicles to transport tanks by road and moves to take advantage of melting ice in the Arctic. “Russia is not sitting around waiting to see what happens.”