Putin said Saturday it was his duty to protect ethnic Russians in Crimea and southeastern swathes of Ukraine that have ancient ties to Moscow and look on Kiev's new pro-EU leaders with disdain.
Russian officials also argued they had no need to turn for permission from the UN Security Council -- as Putin had demanded for any Western action in Syria -- because the well-being of their own citizens was at stake.
NATO chief Anders Fogh Rasmussen, holding urgent talks in Brussels, told Russia to put an immediate end to its military activities, saying it "threatens peace and security in Europe".
German Frank-Walter Steinmeier spoke ominously of preventing "a new division of Europe" while France and Britain called for negotiations to be organised between Moscow and Kiev, either directly or through the United Nations.
In the most immediate response to Russia's actions in the country on the eastern edge of Europe, the U.S. and its Western allies pulled out of preparatory meetings this week for the June G8 summit in the Russian Black Sea resort of Sochi.