NATO would struggle to defend the Baltic states from any Russian aggression "with conventional means", Germany's Spiegel magazine reported on Sunday, citing sources close to the organisation 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 United States 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," the weekly magazine said, citing a NATO defence planning committee document.