"We have worked actively to forge a strategic partnership with Russia" since the Cold War, NATO chief Jens Stoltenberg told a defence and peace meeting in Italy.
"However, the aggressive behaviour of Russia has undermined stability and security in Europe," he added.
The 2014 incursion of pro-Russian forces into eastern Ukraine and Russia's annexation of Crimea have strained ties with the west to the limit.
In response, NATO has bolstered its presence in eastern European countries that are now allies but were once ruled from Moscow.
Stoltenberg described Russia's "illegal annexation of Crimea" as "the first time since World War II that a European nation had taken the territory of another country by force.
"Russia continues to destabilise Eastern Ukraine. It is a conflict in which nearly 10,000 Ukrainians have been killed and this has changed the security context enormously," he said.
But the NATO chief said he was not looking for a confrontation with Russia and that he strongly believed in dialogue.
The alliance has deployed multinational battalions to Estonia, Latvia, Lithuania and Poland as NATO seeks to reassure them they will not be abandoned in any fresh crisis.
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