Five Nordic nations have agreed to work more closely on defense after declaring Russia the "biggest challenge to European security."

The defense ministers of Sweden, Norway, Finland, Denmark and Iceland said Northern Europe must prepare for possible crises or incidents because of Russia.

In a statement published in the daily Aftenposten, the ministers said Russian leaders had shown they are ready to use military means to reach political goals.

They noted increasing Russian military and intelligence activity in the Baltics and other northern European areas.

"The Russian military is challenging us along our borders and there have been several border infringements in the Baltics."

The statement comes amid heightened tensions in Europe since Russia annexed Crimea from Ukraine a year ago.

With large Russian minorities living in the Baltics, concerns have grown in the region about the risk of Russian intervention.

Based on reporting by Reuters