NATO would struggle to defend the Baltic states from any Russian aggression "with conventional means," a news report said, citing sources close to the organization and a draft of a NATO planning document.
Eastern European states are nervous about Russia after it annexed Ukraine's Crimea region and massed 40,000 troops on Ukraine's borders.
The U.S. has sent 600 soldiers to the three Baltic countries — Estonia, Latvia and Lithuania — and Poland to take part in exercises to bolster NATO's presence in eastern Europe.
"Russia's ability and intention to undertake significant military action without much forewarning poses a far-reaching threat for the maintenance of security and stability in the European-Atlantic area," Germany's Spiegel magazine said, citing a NATO defense planning committee document.
Russia is capable of building up a local or regional military threat at short notice and at an arbitrary spot, the draft document continued.
However, Europe at the end of the Cold War had concluded that "it could reduce its capabilities for fighting conventional, large scale and high intensity conflicts in Europe".
"While we never comment on alleged leaks or on our defense plans, NATO's core task is collective defense and we will do what it takes to defend any ally under attack," said NATO spokeswoman Oana Lungescu.
"In light of the new security situation created by Russia's illegal and illegitimate aggression against Ukraine, we have taken immediate measures to enhance collective security in the air, at sea and on the ground."
The Ukraine crisis has compelled the alliance to refocus on its core mission of defending its members after years in which its main effort has been far away in Afghanistan.