An expert panel is warning the Finnish government it could expect "harsh" reactions from Russia if it decided to join NATO, but would be better off doing so together with neighboring Sweden.

The report released Friday added that "specific counter-measures would be difficult to predict."

In Moscow, Russian Foreign Minister Sergey Lavrov warned of a "military-technical" response if its "neutral neighbors" joined the alliance.

NATO supporters in the two non-aligned Nordic countries have stepped up calls for membership following the Ukraine crisis and increased assertiveness by Russian forces around the Baltic Sea. Both Finland and Sweden have deepened their partnership with the alliance in recent years but have so far not sought membership, partly because of concern over the Russian reaction.